Do not add duplicate entries to description dictionary

actions-expressions/src/completion/descriptionDictionary.test.ts

@@ -12,7 +12,8 @@ describe("description dictionary", () => {
   it("does not add duplicate entries", () => {
     const d = new DescriptionDictionary();
     d.add("ABC", new StringData("val1"));
-    d.add("abc", new StringData("val2"));
+    d.add("ABC", new StringData("val2"));
+    d.add("abc", new StringData("val3"));
 
     expect(d.pairs()).toEqual([{key: "ABC", value: new StringData("val1")}]);
   });

actions-expressions/src/completion/descriptionDictionary.ts

@@ -19,6 +19,11 @@ export class DescriptionDictionary extends Dictionary {
   }
 
   override add(key: string, value: ExpressionData, description?: string): void {
+    if (this.get(key) !== undefined) {
+      // Key already added, ignore
+      return;
+    }
+
     super.add(key, value);
     if (description) {
       this.descriptions.set(key, description);