Suggestion for on-responded on prompts

On issue #3947, relying on the responded flag todecide whether to show the prompt interferes with logic that could be happening either

a. Concurrently
b. On some middleware
c. As part of how we reached the prompt

I believe the *intention* is to prevent sending a prompt twice to the user, using a symbol may be enough for that, without relying on global status

libraries/botbuilder-dialogs/src/prompts/prompt.ts

@@ -200,12 +200,15 @@ export abstract class Prompt<T> extends Dialog {
         state.options = opt;
         state.state = {};
 
+        dc.context.turnState.set(this.hasPrompted, true)
         // Send initial prompt
         await this.onPrompt(dc.context, state.state, state.options, false);
 
         return Dialog.EndOfTurn;
     }
 
+    protected hasPrompted = Symbol('hasPrompted')
+
     /**
      * Called when a prompt dialog is the active dialog and the user replied with a new activity.
      * @param dc The [DialogContext](xref:botbuilder-dialogs.DialogContext) for the current turn of conversation.
@@ -257,7 +260,8 @@ export abstract class Prompt<T> extends Dialog {
         if (isValid) {
             return await dc.endDialog(recognized.value);
         } else {
-            if (!dc.context.responded) {
+            if (!dc.context.turnState.get(this.hasPrompted)) {
+                dc.context.turnState.set(this.hasPrompted, true)
                 await this.onPrompt(dc.context, state.state, state.options, true);
             }