diff --git a/src/scope_stack.h b/src/scope_stack.h
index 5e8b57ef50..f127c97903 100644
--- a/src/scope_stack.h
+++ b/src/scope_stack.h
@@ -64,14 +64,27 @@ class ScopeStack {
   /// @param ret where to place the name
   /// @returns true if the name was successfully found, false otherwise
   bool get(const std::string& name, T* ret) const {
+    return get(name, ret, nullptr);
+  }
+
+  /// Retrieves a given name from the stack
+  /// @param name the name to look for
+  /// @param ret where to place the name
+  /// @param is_global set true if the name references a global variable
+  /// otherwise unchanged
+  /// @returns true if the name was successfully found, false otherwise
+  bool get(const std::string& name, T* ret, bool* is_global) const {
     for (auto iter = stack_.rbegin(); iter != stack_.rend(); ++iter) {
       auto& map = *iter;
-
       auto val = map.find(name);
+
       if (val != map.end()) {
         if (ret) {
           *ret = val->second;
         }
+        if (is_global && iter == stack_.rend() - 1) {
+          *is_global = true;
+        }
         return true;
       }
     }
diff --git a/src/validator_impl.cc b/src/validator_impl.cc
index da7908c436..b3301964d3 100644
--- a/src/validator_impl.cc
+++ b/src/validator_impl.cc
@@ -31,6 +31,11 @@ bool ValidatorImpl::Validate(const ast::Module* module) {
     return false;
   }
   for (const auto& var : module->global_variables()) {
+    if (variable_stack_.has(var->name())) {
+      set_error(var->source(),
+                "v-0011: redeclared global identifier '" + var->name() + "'");
+      return false;
+    }
     variable_stack_.set_global(var->name(), var.get());
   }
   if (!CheckImports(module)) {
@@ -70,11 +75,6 @@ bool ValidatorImpl::ValidateStatements(const ast::BlockStatement* block) {
     return false;
   }
   for (const auto& stmt : *block) {
-    // TODO(sarahM0): move the folowing to a function
-    if (stmt->IsVariableDecl()) {
-      auto* v = stmt->AsVariableDecl();
-      variable_stack_.set(v->variable()->name(), v->variable());
-    }
     if (!ValidateStatement(stmt.get())) {
       return false;
     }
@@ -82,10 +82,30 @@ bool ValidatorImpl::ValidateStatements(const ast::BlockStatement* block) {
   return true;
 }
 
+bool ValidatorImpl::ValidateDeclStatement(
+    const ast::VariableDeclStatement* decl) {
+  auto name = decl->variable()->name();
+  bool is_global = false;
+  if (variable_stack_.get(name, nullptr, &is_global)) {
+    std::string error_number = "v-0014: ";
+    if (is_global) {
+      error_number = "v-0013: ";
+    }
+    set_error(decl->source(),
+              error_number + "redeclared identifier '" + name + "'");
+    return false;
+  }
+  variable_stack_.set(name, decl->variable());
+  return true;
+}
+
 bool ValidatorImpl::ValidateStatement(const ast::Statement* stmt) {
   if (!stmt) {
     return false;
   }
+  if (stmt->IsVariableDecl()) {
+    return ValidateDeclStatement(stmt->AsVariableDecl());
+  }
   if (stmt->IsAssign()) {
     return ValidateAssign(stmt->AsAssign());
   }
diff --git a/src/validator_impl.h b/src/validator_impl.h
index d7a7fc3227..0019d7da87 100644
--- a/src/validator_impl.h
+++ b/src/validator_impl.h
@@ -89,6 +89,11 @@ class ValidatorImpl {
   /// @param assign is the assigment to check if its lhs is a const
   /// @returns false if lhs of assign is a constant identifier
   bool ValidateConstant(const ast::AssignmentStatement* assign);
+  /// Validates declaration name uniquness
+  /// @param decl is the new declartion to be added
+  /// @returns true if no previous decleration with the |decl|'s name
+  /// exist in the variable stack
+  bool ValidateDeclStatement(const ast::VariableDeclStatement* decl);
 
  private:
   std::string error_;
diff --git a/src/validator_test.cc b/src/validator_test.cc
index cb5d6dbd63..0a116b9ea5 100644
--- a/src/validator_test.cc
+++ b/src/validator_test.cc
@@ -48,6 +48,7 @@
 #include "src/ast/type/pointer_type.h"
 #include "src/ast/type/struct_type.h"
 #include "src/ast/type/vector_type.h"
+#include "src/ast/type/void_type.h"
 #include "src/ast/type_constructor_expression.h"
 #include "src/ast/variable.h"
 #include "src/ast/variable_decl_statement.h"
@@ -240,7 +241,7 @@ TEST_F(ValidatorTest, AssignCompatibleTypesInBlockStatement_Pass) {
   ASSERT_NE(rhs_ptr->result_type(), nullptr);
 
   tint::ValidatorImpl v;
-  EXPECT_TRUE(v.ValidateStatements(body.get()));
+  EXPECT_TRUE(v.ValidateStatements(body.get())) << v.error();
 }
 
 TEST_F(ValidatorTest, AssignIncompatibleTypesInBlockStatement_Fail) {
@@ -278,7 +279,11 @@ TEST_F(ValidatorTest, AssignIncompatibleTypesInBlockStatement_Fail) {
             "12:34: v-000x: invalid assignment of '__i32' to '__f32'");
 }
 
-TEST_F(ValidatorTest, UnsingUndefinedVariableGlobalVariable_Fail) {
+TEST_F(ValidatorTest, UsingUndefinedVariableGlobalVariable_Fail) {
+  // var global_var: f32 = 2.1;
+  // fn my_func() -> f32 {
+  //   not_global_var = 3.14f;
+  // }
   ast::type::F32Type f32;
   auto global_var = std::make_unique<ast::Variable>(
       "global_var", ast::StorageClass::kPrivate, &f32);
@@ -307,7 +312,11 @@ TEST_F(ValidatorTest, UnsingUndefinedVariableGlobalVariable_Fail) {
   EXPECT_EQ(v.error(), "12:34: v-0006: 'not_global_var' is not declared");
 }
 
-TEST_F(ValidatorTest, UnsingUndefinedVariableGlobalVariable_Pass) {
+TEST_F(ValidatorTest, UsingUndefinedVariableGlobalVariable_Pass) {
+  // var global_var: f32 = 2.1;
+  // fn my_func() -> f32 {
+  //   global_var = 3.14;
+  // }
   ast::type::F32Type f32;
   auto global_var = std::make_unique<ast::Variable>(
       "global_var", ast::StorageClass::kPrivate, &f32);
@@ -341,10 +350,10 @@ TEST_F(ValidatorTest, UnsingUndefinedVariableGlobalVariable_Pass) {
   EXPECT_TRUE(v.Validate(mod())) << v.error();
 }
 
-TEST_F(ValidatorTest, UnsingUndefinedVariableInnerScope_Fail) {
+TEST_F(ValidatorTest, UsingUndefinedVariableInnerScope_Fail) {
   // {
-  // if (true) { var a : f32 = 2.0; }
-  // a = 3.14;
+  //   if (true) { var a : f32 = 2.0; }
+  //   a = 3.14;
   // }
   ast::type::F32Type f32;
   auto var =
@@ -378,10 +387,10 @@ TEST_F(ValidatorTest, UnsingUndefinedVariableInnerScope_Fail) {
   EXPECT_EQ(v.error(), "12:34: v-0006: 'a' is not declared");
 }
 
-TEST_F(ValidatorTest, UnsingUndefinedVariableOuterScope_Pass) {
+TEST_F(ValidatorTest, UsingUndefinedVariableOuterScope_Pass) {
   // {
-  // var a : f32 = 2.0;
-  // if (true) { a = 3.14; }
+  //   var a : f32 = 2.0;
+  //   if (true) { a = 3.14; }
   // }
   ast::type::F32Type f32;
   auto var =
@@ -413,6 +422,27 @@ TEST_F(ValidatorTest, UnsingUndefinedVariableOuterScope_Pass) {
   EXPECT_TRUE(v.ValidateStatements(outer_body.get())) << v.error();
 }
 
+TEST_F(ValidatorTest, GlobalVariableUnique_Pass) {
+  // var global_var0 : f32 = 0.1;
+  // var global_var1 : i32 = 0;
+  ast::type::F32Type f32;
+  ast::type::I32Type i32;
+  auto var0 = std::make_unique<ast::Variable>(
+      "global_var0", ast::StorageClass::kPrivate, &f32);
+  var0->set_constructor(std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::FloatLiteral>(&f32, 0.1)));
+  mod()->AddGlobalVariable(std::move(var0));
+
+  auto var1 = std::make_unique<ast::Variable>(
+      Source{12, 34}, "global_var1", ast::StorageClass::kPrivate, &f32);
+  var1->set_constructor(std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::SintLiteral>(&i32, 0)));
+  mod()->AddGlobalVariable(std::move(var1));
+
+  tint::ValidatorImpl v;
+  EXPECT_TRUE(v.Validate(mod())) << v.error();
+}
+
 TEST_F(ValidatorTest, AssignToConstant_Fail) {
   // {
   //  const a :i32 = 2;
@@ -445,5 +475,213 @@ TEST_F(ValidatorTest, AssignToConstant_Fail) {
   EXPECT_EQ(v.error(), "12:34: v-0021: cannot re-assign a constant: 'a'");
 }
 
+TEST_F(ValidatorTest, GlobalVariableNotUnique_Fail) {
+  // var global_var : f32 = 0.1;
+  // var global_var : i32 = 0;
+  ast::type::F32Type f32;
+  ast::type::I32Type i32;
+  auto var0 = std::make_unique<ast::Variable>(
+      "global_var", ast::StorageClass::kPrivate, &f32);
+  var0->set_constructor(std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::FloatLiteral>(&f32, 0.1)));
+  mod()->AddGlobalVariable(std::move(var0));
+
+  auto var1 = std::make_unique<ast::Variable>(
+      Source{12, 34}, "global_var", ast::StorageClass::kPrivate, &f32);
+  var1->set_constructor(std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::SintLiteral>(&i32, 0)));
+  mod()->AddGlobalVariable(std::move(var1));
+
+  tint::ValidatorImpl v;
+  EXPECT_FALSE(v.Validate(mod()));
+  EXPECT_EQ(v.error(),
+            "12:34: v-0011: redeclared global identifier 'global_var'");
+}
+
+TEST_F(ValidatorTest, GlobalVariableFunctionVariableNotUnique_Fail) {
+  // var a: f32 = 2.1;
+  // fn my_func -> void {
+  //   var a: f32 = 2.0;
+  //   return 0;
+  // }
+
+  ast::type::VoidType void_type;
+  ast::type::F32Type f32;
+  auto global_var =
+      std::make_unique<ast::Variable>("a", ast::StorageClass::kPrivate, &f32);
+  global_var->set_constructor(
+      std::make_unique<ast::ScalarConstructorExpression>(
+          std::make_unique<ast::FloatLiteral>(&f32, 2.1)));
+  mod()->AddGlobalVariable(std::move(global_var));
+
+  auto var =
+      std::make_unique<ast::Variable>("a", ast::StorageClass::kNone, &f32);
+  var->set_constructor(std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::FloatLiteral>(&f32, 2.0)));
+  ast::VariableList params;
+  auto func =
+      std::make_unique<ast::Function>("my_func", std::move(params), &void_type);
+  auto body = std::make_unique<ast::BlockStatement>();
+  body->append(std::make_unique<ast::VariableDeclStatement>(Source{12, 34},
+                                                            std::move(var)));
+  func->set_body(std::move(body));
+  auto* func_ptr = func.get();
+  mod()->AddFunction(std::move(func));
+
+  EXPECT_TRUE(td()->Determine()) << td()->error();
+  EXPECT_TRUE(td()->DetermineFunction(func_ptr)) << td()->error();
+  tint::ValidatorImpl v;
+  EXPECT_FALSE(v.Validate(mod())) << v.error();
+  EXPECT_EQ(v.error(), "12:34: v-0013: redeclared identifier 'a'");
+}
+
+TEST_F(ValidatorTest, RedeclaredIndentifier_Fail) {
+  // fn my_func() -> void {
+  //  var a :i32 = 2;
+  //  var a :f21 = 2.0;
+  // }
+  ast::type::VoidType void_type;
+  ast::type::I32Type i32;
+  ast::type::F32Type f32;
+  auto var =
+      std::make_unique<ast::Variable>("a", ast::StorageClass::kNone, &i32);
+  var->set_constructor(std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::SintLiteral>(&i32, 2)));
+
+  auto var_a_float =
+      std::make_unique<ast::Variable>("a", ast::StorageClass::kNone, &f32);
+  var_a_float->set_constructor(
+      std::make_unique<ast::ScalarConstructorExpression>(
+          std::make_unique<ast::FloatLiteral>(&f32, 0.1)));
+
+  ast::VariableList params;
+  auto func =
+      std::make_unique<ast::Function>("my_func", std::move(params), &void_type);
+  auto body = std::make_unique<ast::BlockStatement>();
+  body->append(std::make_unique<ast::VariableDeclStatement>(std::move(var)));
+  body->append(std::make_unique<ast::VariableDeclStatement>(
+      Source{12, 34}, std::move(var_a_float)));
+  func->set_body(std::move(body));
+  auto* func_ptr = func.get();
+  mod()->AddFunction(std::move(func));
+
+  EXPECT_TRUE(td()->Determine()) << td()->error();
+  EXPECT_TRUE(td()->DetermineFunction(func_ptr)) << td()->error();
+  tint::ValidatorImpl v;
+  EXPECT_FALSE(v.Validate(mod()));
+  EXPECT_EQ(v.error(), "12:34: v-0014: redeclared identifier 'a'");
+}
+
+TEST_F(ValidatorTest, RedeclaredIdentifierInnerScope_Pass) {
+  // {
+  // if (true) { var a : f32 = 2.0; }
+  // var a : f32 = 3.14;
+  // }
+  ast::type::F32Type f32;
+  auto var =
+      std::make_unique<ast::Variable>("a", ast::StorageClass::kNone, &f32);
+  var->set_constructor(std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::FloatLiteral>(&f32, 2.0)));
+
+  ast::type::BoolType bool_type;
+  auto cond = std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::BoolLiteral>(&bool_type, true));
+  auto body = std::make_unique<ast::BlockStatement>();
+  body->append(std::make_unique<ast::VariableDeclStatement>(std::move(var)));
+
+  auto var_a_float =
+      std::make_unique<ast::Variable>("a", ast::StorageClass::kNone, &f32);
+  var_a_float->set_constructor(
+      std::make_unique<ast::ScalarConstructorExpression>(
+          std::make_unique<ast::FloatLiteral>(&f32, 3.14)));
+
+  auto outer_body = std::make_unique<ast::BlockStatement>();
+  outer_body->append(
+      std::make_unique<ast::IfStatement>(std::move(cond), std::move(body)));
+  outer_body->append(std::make_unique<ast::VariableDeclStatement>(
+      Source{12, 34}, std::move(var_a_float)));
+
+  EXPECT_TRUE(td()->DetermineStatements(outer_body.get())) << td()->error();
+  tint::ValidatorImpl v;
+  EXPECT_TRUE(v.ValidateStatements(outer_body.get())) << v.error();
+}
+
+TEST_F(ValidatorTest, DISABLED_RedeclaredIdentifierInnerScope_False) {
+  // TODO(sarahM0): remove DISABLED after implementing ValidateIfStatement
+  // and it should just work
+  // {
+  // var a : f32 = 3.14;
+  // if (true) { var a : f32 = 2.0; }
+  // }
+  ast::type::F32Type f32;
+  auto var_a_float =
+      std::make_unique<ast::Variable>("a", ast::StorageClass::kNone, &f32);
+  var_a_float->set_constructor(
+      std::make_unique<ast::ScalarConstructorExpression>(
+          std::make_unique<ast::FloatLiteral>(&f32, 3.14)));
+
+  auto var =
+      std::make_unique<ast::Variable>("a", ast::StorageClass::kNone, &f32);
+  var->set_constructor(std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::FloatLiteral>(&f32, 2.0)));
+
+  ast::type::BoolType bool_type;
+  auto cond = std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::BoolLiteral>(&bool_type, true));
+  auto body = std::make_unique<ast::BlockStatement>();
+  body->append(std::make_unique<ast::VariableDeclStatement>(Source{12, 34},
+                                                            std::move(var)));
+
+  auto outer_body = std::make_unique<ast::BlockStatement>();
+  outer_body->append(
+      std::make_unique<ast::VariableDeclStatement>(std::move(var_a_float)));
+  outer_body->append(
+      std::make_unique<ast::IfStatement>(std::move(cond), std::move(body)));
+
+  EXPECT_TRUE(td()->DetermineStatements(outer_body.get())) << td()->error();
+  tint::ValidatorImpl v;
+  EXPECT_FALSE(v.ValidateStatements(outer_body.get()));
+  EXPECT_EQ(v.error(), "12:34: v-0014: redeclared identifier 'a'");
+}
+
+TEST_F(ValidatorTest, RedeclaredIdentifierDifferentFunctions_Pass) {
+  // func0 { var a : f32 = 2.0; }
+  // func1 { var a : f32 = 3.0; }
+  ast::type::F32Type f32;
+  auto var0 =
+      std::make_unique<ast::Variable>("a", ast::StorageClass::kNone, &f32);
+  var0->set_constructor(std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::FloatLiteral>(&f32, 2.0)));
+
+  auto var1 =
+      std::make_unique<ast::Variable>("a", ast::StorageClass::kNone, &f32);
+  var1->set_constructor(std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::FloatLiteral>(&f32, 1.0)));
+
+  ast::VariableList params0;
+  auto func0 =
+      std::make_unique<ast::Function>("func0", std::move(params0), &f32);
+  auto body0 = std::make_unique<ast::BlockStatement>();
+  body0->append(std::make_unique<ast::VariableDeclStatement>(Source{12, 34},
+                                                             std::move(var0)));
+  func0->set_body(std::move(body0));
+
+  ast::VariableList params1;
+  auto func1 =
+      std::make_unique<ast::Function>("func1", std::move(params1), &f32);
+  auto body1 = std::make_unique<ast::BlockStatement>();
+  body1->append(std::make_unique<ast::VariableDeclStatement>(Source{13, 34},
+                                                             std::move(var1)));
+  func1->set_body(std::move(body1));
+
+  mod()->AddFunction(std::move(func0));
+  mod()->AddFunction(std::move(func1));
+
+  EXPECT_TRUE(td()->Determine()) << td()->error();
+  EXPECT_TRUE(td()->Determine()) << td()->error();
+  tint::ValidatorImpl v;
+  EXPECT_TRUE(v.Validate(mod())) << v.error();
+}
+
 }  // namespace
 }  // namespace tint