Offer missing abstract codefix once

* per class that is missing potentially many abstract members.

src/harness/fourslash.ts

@@ -2153,12 +2153,29 @@ namespace FourSlash {
             const diagnostics: ts.Diagnostic[] = this.getDiagnostics(fileName);
 
             let actions: ts.CodeAction[] = undefined;
+
+            const checkedDiagnostics = ts.createMap<boolean>();
+
             for (const diagnostic of diagnostics) {
 
                 if (errorCode && errorCode !== diagnostic.code) {
                     continue;
                 }
 
+                const summary = JSON.stringify({
+                    start: diagnostic.start,
+                    length: diagnostic.length,
+                    code: diagnostic.code
+                });
+
+                if (checkedDiagnostics.has(summary)) {
+                    // Don't want to ask for code fixes in an identical position for the same error code twice.
+                    continue;
+                }
+                else {
+                    checkedDiagnostics.set(summary, true);
+                }
+
                 const newActions = this.languageService.getCodeFixesAtPosition(fileName, diagnostic.start, diagnostic.length, [diagnostic.code]);
                 if (newActions && newActions.length) {
                     actions = actions ? actions.concat(newActions) : newActions;

src/services/services.ts

@@ -1688,7 +1688,13 @@ namespace ts {
 
             let allFixes: CodeAction[] = [];
 
-            forEach(errorCodes, error => {
+            // De-duplicate error codes.
+            const errorCodeSet: number[] = [];
+            for (const code of errorCodes) {
+                errorCodeSet[code] = code;
+            }
+
+            forEach(errorCodeSet, error => {
                 cancellationToken.throwIfCancellationRequested();
 
                 const context = {

tests/cases/fourslash/codeFixClassExtendAbstractProperty.ts

@@ -2,11 +2,15 @@
 
 //// abstract class A {
 ////    abstract x: number;
+////    abstract foo(): number;
 //// }
 ////
 //// class C extends A {[|
 //// |]}
 
 verify.rangeAfterCodeFix(`
-x: number;
+    x: number;
+    foo(): number {
+        throw new Error('Method not implemented.');
+    }
 `);