diff --git a/NOTICE b/NOTICE
index 6b4450d592..6226bc34ab 100644
--- a/NOTICE
+++ b/NOTICE
@@ -42,6 +42,7 @@ under the licensing terms detailed in LICENSE:
 * Roman F. <70765447+romdotdog@users.noreply.github.com>
 * Joe Pea <trusktr@gmail.com>
 * Felipe Gasper <FGasper@users.noreply.github.com>
+* Congcong Cai <77575210+HerrCai0907@users.noreply.github.com>
 
 Portions of this software are derived from third-party works licensed under
 the following terms:
diff --git a/src/compiler.ts b/src/compiler.ts
index a0dd74d9e6..e035cab8ca 100644
--- a/src/compiler.ts
+++ b/src/compiler.ts
@@ -3030,7 +3030,7 @@ export class Compiler extends DiagnosticEmitter {
 
         if (initializerNode) {
           let pendingElements = this.pendingElements;
-          let dummy = flow.addScopedDummyLocal(name, type); // pending dummy
+          let dummy = flow.addScopedDummyLocal(name, type, statement); // pending dummy
           pendingElements.add(dummy);
           initExpr = this.compileExpression(initializerNode, type, // reports
             Constraints.CONV_IMPLICIT
@@ -3042,7 +3042,7 @@ export class Compiler extends DiagnosticEmitter {
       // Otherwise infer type from initializer
       } else if (initializerNode) {
         let pendingElements = this.pendingElements;
-        let temp = flow.addScopedDummyLocal(name, Type.auto); // pending dummy
+        let temp = flow.addScopedDummyLocal(name, Type.auto, statement); // pending dummy
         pendingElements.add(temp);
         initExpr = this.compileExpression(initializerNode, Type.auto); // reports
         pendingElements.delete(temp);
diff --git a/src/diagnosticMessages.generated.ts b/src/diagnosticMessages.generated.ts
index 0f0f3a00d1..77d2012864 100644
--- a/src/diagnosticMessages.generated.ts
+++ b/src/diagnosticMessages.generated.ts
@@ -151,6 +151,7 @@ export enum DiagnosticCode {
   A_namespace_declaration_cannot_be_located_prior_to_a_class_or_function_with_which_it_is_merged = 2434,
   Property_0_is_protected_and_only_accessible_within_class_1_and_its_subclasses = 2445,
   Variable_0_used_before_its_declaration = 2448,
+  Cannot_redeclare_block_scoped_variable_0 = 2451,
   The_type_argument_for_type_parameter_0_cannot_be_inferred_from_the_usage_Consider_specifying_the_type_arguments_explicitly = 2453,
   Type_0_has_no_property_1 = 2460,
   The_0_operator_cannot_be_applied_to_type_1 = 2469,
@@ -333,6 +334,7 @@ export function diagnosticCodeToString(code: DiagnosticCode): string {
     case 2434: return "A namespace declaration cannot be located prior to a class or function with which it is merged.";
     case 2445: return "Property '{0}' is protected and only accessible within class '{1}' and its subclasses.";
     case 2448: return "Variable '{0}' used before its declaration.";
+    case 2451: return "Cannot redeclare block-scoped variable '{0}'";
     case 2453: return "The type argument for type parameter '{0}' cannot be inferred from the usage. Consider specifying the type arguments explicitly.";
     case 2460: return "Type '{0}' has no property '{1}'.";
     case 2469: return "The '{0}' operator cannot be applied to type '{1}'.";
diff --git a/src/diagnosticMessages.json b/src/diagnosticMessages.json
index 8a76e1eb12..7975f441d2 100644
--- a/src/diagnosticMessages.json
+++ b/src/diagnosticMessages.json
@@ -149,6 +149,7 @@
   "A namespace declaration cannot be located prior to a class or function with which it is merged.": 2434,
   "Property '{0}' is protected and only accessible within class '{1}' and its subclasses.": 2445,
   "Variable '{0}' used before its declaration.": 2448,
+  "Cannot redeclare block-scoped variable '{0}'" : 2451,
   "The type argument for type parameter '{0}' cannot be inferred from the usage. Consider specifying the type arguments explicitly.": 2453,
   "Type '{0}' has no property '{1}'.": 2460,
   "The '{0}' operator cannot be applied to type '{1}'.": 2469,
diff --git a/src/flow.ts b/src/flow.ts
index 100d97ac8e..ee8671ee8f 100644
--- a/src/flow.ts
+++ b/src/flow.ts
@@ -444,11 +444,16 @@ export class Flow {
   }
 
   /** Adds a new scoped dummy local of the specified name. */
-  addScopedDummyLocal(name: string, type: Type): Local {
+  addScopedDummyLocal(name: string, type: Type, declarationNode: Node): Local {
     var scopedDummy = new Local(name, -1, type, this.parentFunction);
     var scopedLocals = this.scopedLocals;
     if (!scopedLocals) this.scopedLocals = scopedLocals = new Map();
-    else assert(!scopedLocals.has(name));
+    else if (scopedLocals.has(name)) {
+      this.parentFunction.program.error(
+        DiagnosticCode.Cannot_redeclare_block_scoped_variable_0,
+        declarationNode.range, name
+      );
+    }
     scopedDummy.set(CommonFlags.SCOPED);
     scopedLocals.set(name, scopedDummy);
     return scopedDummy;