Don't add AppSync PubSub provider more than once

[manueliglesias]

Issue #, if available:

Description of changes:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[elorzafe]

👍

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@0861e34). Click here to learn what that means.
The diff coverage is 50%.

@@            Coverage Diff            @@
##             master     #649   +/-   ##
=========================================
  Coverage          ?   86.93%           
=========================================
  Files             ?       71           
  Lines             ?     3345           
  Branches          ?      634           
=========================================
  Hits              ?     2908           
  Misses            ?      420           
  Partials          ?       17

Impacted Files	Coverage Δ
packages/aws-amplify/src/PubSub/PubSub.ts	91.17% <50%> (ø)

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[github-actions]

This pull request has been automatically locked since there hasn't been any recent activity after it was closed. Please open a new issue for related bugs.

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