fix: in group creation don't show duplicate error message when members added again

[marlonkeating]

Jira Ticket

This change makes it so that when members are repeatedly added to a group from the member table, a duplicate error is not shown.

Testing Instructions

• Go to the admin-portal repo, pull down branch, and run npm run start:stage
• Navigate to https://localhost.stage.edx.org:1991/alc-general/admin/people-management
• Click 'Create group'
• Check one member and click 'Add' button
• While leaving first member checked, check another member in the table, click 'Add' again
• Verify that both members are shown in the Summary section, and there is no error message indicating duplicate members have been added.

For all changes

• Ensure adequate tests are in place (or reviewed existing tests cover changes)

Only if submitting a visual change

• Ensure to attach screenshots
• Ensure to have UX team confirm screenshots

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.46%. Comparing base (c3e9edf) to head (26b4323).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1425   +/-   ##
=======================================
  Coverage   86.46%   86.46%           
=======================================
  Files         660      660           
  Lines       14921    14921           
  Branches     3159     3163    +4     
=======================================
  Hits        12902    12902           
  Misses       1949     1949           
  Partials       70       70           

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