diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 664a84ec99a97..2b951a2e60abb 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7772,7 +7772,7 @@ namespace ts {
         }
 
         function createUnionOrIntersectionProperty(containingType: UnionOrIntersectionType, name: __String): Symbol | undefined {
-            let props: Symbol[] | undefined;
+            const propSet = createMap<Symbol>();
             let indexTypes: Type[] | undefined;
             const isUnion = containingType.flags & TypeFlags.Union;
             const excludeModifiers = isUnion ? ModifierFlags.NonPublicAccessibilityModifier : 0;
@@ -7787,7 +7787,10 @@ namespace ts {
                     const modifiers = prop ? getDeclarationModifierFlagsFromSymbol(prop) : 0;
                     if (prop && !(modifiers & excludeModifiers)) {
                         commonFlags &= prop.flags;
-                        props = appendIfUnique(props, prop);
+                        const id = "" + getSymbolId(prop);
+                        if (!propSet.has(id)) {
+                            propSet.set(id, prop);
+                        }
                         checkFlags |= (isReadonlySymbol(prop) ? CheckFlags.Readonly : 0) |
                             (!(modifiers & ModifierFlags.NonPublicAccessibilityModifier) ? CheckFlags.ContainsPublic : 0) |
                             (modifiers & ModifierFlags.Protected ? CheckFlags.ContainsProtected : 0) |
@@ -7809,9 +7812,10 @@ namespace ts {
                     }
                 }
             }
-            if (!props) {
+            if (!propSet.size) {
                 return undefined;
             }
+            const props = arrayFrom(propSet.values());
             if (props.length === 1 && !(checkFlags & CheckFlags.Partial) && !indexTypes) {
                 return props[0];
             }