diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index f901db2ab4c5c..bf198f02e911b 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -1320,13 +1320,14 @@ namespace ts {
             else { // error
                 const isEitherEnum = !!(target.flags & SymbolFlags.Enum || source.flags & SymbolFlags.Enum);
                 const isEitherBlockScoped = !!(target.flags & SymbolFlags.BlockScopedVariable || source.flags & SymbolFlags.BlockScopedVariable);
-                const message = isEitherEnum
-                    ? Diagnostics.Enum_declarations_can_only_merge_with_namespace_or_other_enum_declarations
-                    : isEitherBlockScoped
-                        ? Diagnostics.Cannot_redeclare_block_scoped_variable_0
-                        : Diagnostics.Duplicate_identifier_0;
+                const message = isEitherEnum ? Diagnostics.Enum_declarations_can_only_merge_with_namespace_or_other_enum_declarations
+                    : isEitherBlockScoped ? Diagnostics.Cannot_redeclare_block_scoped_variable_0
+                    : Diagnostics.Duplicate_identifier_0;
                 const sourceSymbolFile = source.declarations && getSourceFileOfNode(source.declarations[0]);
                 const targetSymbolFile = target.declarations && getSourceFileOfNode(target.declarations[0]);
+
+                const isSourcePlainJs = isPlainJsFile(sourceSymbolFile, compilerOptions.checkJs);
+                const isTargetPlainJs = isPlainJsFile(targetSymbolFile, compilerOptions.checkJs);
                 const symbolName = symbolToString(source);
 
                 // Collect top-level duplicate identifier errors into one mapping, so we can then merge their diagnostics if there are a bunch
@@ -1337,12 +1338,12 @@ namespace ts {
                         ({ firstFile, secondFile, conflictingSymbols: new Map() } as DuplicateInfoForFiles));
                     const conflictingSymbolInfo = getOrUpdate(filesDuplicates.conflictingSymbols, symbolName, () =>
                         ({ isBlockScoped: isEitherBlockScoped, firstFileLocations: [], secondFileLocations: [] } as DuplicateInfoForSymbol));
-                    addDuplicateLocations(conflictingSymbolInfo.firstFileLocations, source);
-                    addDuplicateLocations(conflictingSymbolInfo.secondFileLocations, target);
+                    if (!isSourcePlainJs) addDuplicateLocations(conflictingSymbolInfo.firstFileLocations, source);
+                    if (!isTargetPlainJs) addDuplicateLocations(conflictingSymbolInfo.secondFileLocations, target);
                 }
                 else {
-                    addDuplicateDeclarationErrorsForSymbols(source, message, symbolName, target);
-                    addDuplicateDeclarationErrorsForSymbols(target, message, symbolName, source);
+                    if (!isSourcePlainJs) addDuplicateDeclarationErrorsForSymbols(source, message, symbolName, target);
+                    if (!isTargetPlainJs) addDuplicateDeclarationErrorsForSymbols(target, message, symbolName, source);
                 }
             }
             return target;
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index f6cacfe719881..d8d22046674dc 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -2102,7 +2102,7 @@ namespace ts {
 
                 const isJs = sourceFile.scriptKind === ScriptKind.JS || sourceFile.scriptKind === ScriptKind.JSX;
                 const isCheckJs = isJs && isCheckJsEnabledForFile(sourceFile, options);
-                const isPlainJs = isJs && !sourceFile.checkJsDirective && options.checkJs === undefined;
+                const isPlainJs = isPlainJsFile(sourceFile, options.checkJs);
                 const isTsNoCheck = !!sourceFile.checkJsDirective && sourceFile.checkJsDirective.enabled === false;
 
                 // By default, only type-check .ts, .tsx, Deferred, plain JS, checked JS and External
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index cabcb2f9b6cac..186df09456d14 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -271,6 +271,10 @@ namespace ts {
         return getSourceFileOfNode(module.valueDeclaration || getNonAugmentationDeclaration(module));
     }
 
+    export function isPlainJsFile(file: SourceFile | undefined, checkJs: boolean | undefined): boolean {
+        return !!file && (file.scriptKind === ScriptKind.JS || file.scriptKind === ScriptKind.JSX) && !file.checkJsDirective && checkJs === undefined;
+    }
+
     export function isStatementWithLocals(node: Node) {
         switch (node.kind) {
             case SyntaxKind.Block:
diff --git a/tests/baselines/reference/plainJSReservedStrict.errors.txt b/tests/baselines/reference/plainJSReservedStrict.errors.txt
index a040fce591c17..db4714a10c194 100644
--- a/tests/baselines/reference/plainJSReservedStrict.errors.txt
+++ b/tests/baselines/reference/plainJSReservedStrict.errors.txt
@@ -1,16 +1,12 @@
 tests/cases/conformance/salsa/plainJSReservedStrict.js(2,7): error TS1100: Invalid use of 'eval' in strict mode.
-tests/cases/conformance/salsa/plainJSReservedStrict.js(2,7): error TS2451: Cannot redeclare block-scoped variable 'eval'.
 tests/cases/conformance/salsa/plainJSReservedStrict.js(3,7): error TS1100: Invalid use of 'arguments' in strict mode.
 
 
-==== tests/cases/conformance/salsa/plainJSReservedStrict.js (3 errors) ====
+==== tests/cases/conformance/salsa/plainJSReservedStrict.js (2 errors) ====
     "use strict"
     const eval = 1
           ~~~~
 !!! error TS1100: Invalid use of 'eval' in strict mode.
-          ~~~~
-!!! error TS2451: Cannot redeclare block-scoped variable 'eval'.
-!!! related TS6203 /.ts/lib.es5.d.ts:32:18: 'eval' was also declared here.
     const arguments = 2
           ~~~~~~~~~
 !!! error TS1100: Invalid use of 'arguments' in strict mode.