[botbuilder] TeamsActivityHandler — Prevent calling event handlers twice

Fixes microsoft#4202

 Description

This change prevents calling TeamsActivityHandler event handlers twice for every event received.

 Specific Changes

Why were the event handlers getting called twice?

- teamsActivityHandler overwrites `dispatchEventActivity`
- `dispatchEventActivity` is only called from within `onEventActivity`
- `onEventActivity` already dispatches *after* calling all handlers
- on teamsActivityHandler, when overwriting, it is also calling all handlers

How to fix it?

- Prevent the handler call inside the dispatchEventHandler
- This should have no effect on any consumer, since they should already be consuming only from within a onEventActivity call.

 Testing

A test is added to prevent the behavior from reappearing

Note that to test the issue described, it is enough to run the changed test in main.
The test will fail when run in main currently, since since it is called twice.

libraries/botbuilder/src/teamsActivityHandler.ts

@@ -1012,18 +1012,16 @@ export class TeamsActivityHandler extends ActivityHandler {
      * custom event sub-type events.
      */
     protected async dispatchEventActivity(context: TurnContext): Promise<void> {
-        await this.handle(context, 'Event', async () => {
-            if (context.activity.channelId === Channels.Msteams) {
-                switch (context.activity.name) {
-                    case 'application/vnd.microsoft.meetingStart':
-                        return this.onTeamsMeetingStart(context);
-                    case 'application/vnd.microsoft.meetingEnd':
-                        return this.onTeamsMeetingEnd(context);
-                }
+        if (context.activity.channelId === Channels.Msteams) {
+            switch (context.activity.name) {
+                case 'application/vnd.microsoft.meetingStart':
+                    return this.onTeamsMeetingStart(context);
+                case 'application/vnd.microsoft.meetingEnd':
+                    return this.onTeamsMeetingEnd(context);
             }
+        }
 
-            return super.dispatchEventActivity(context);
-        });
+        return super.dispatchEventActivity(context);
     }
 
     /**

libraries/botbuilder/tests/teamsActivityHandler.test.js

@@ -2307,5 +2307,28 @@ describe('TeamsActivityHandler', function () {
                 })
                 .startTest();
         });
+
+        it('should not call middleware handler twice', async function () {
+            const bot = new TeamsActivityHandler();
+
+            const activity = createMeetingEventActivity(false);
+            let ncalls = 0;
+
+            bot.onEvent(async (context, next) => {
+                ncalls++;
+                await next();
+            });
+
+            const adapter = new TestAdapter(async (context) => {
+                await bot.run(context);
+            });
+
+            await adapter
+                .send(activity)
+                .then(() => {
+                    assert(ncalls === 1, 'On Event should only be called once, times called: ' + ncalls.toString());
+                })
+                .startTest();
+        });
     });
 });