500 when adding user twice to group

Started PUT "/me/requests/269344?mark=approve&state=pending" for 127.0.0.1 at 2019-05-08 07:53:17 -0700
Processing by Me::RequestsController#update as JS
  Parameters: {"authenticity_token"=>"...==", "mark"=>"approve", "state"=>"pending", "id"=>"269344"}
Rescuing from AssociationError.
app/models/user/groups.rb:164:in `check_duplicate_memberships'
app/models/group/users.rb:112:in `check_duplicate_memberships'
app/models/group/users.rb:142:in `add_user!'
app/models/requests/membership_request.rb:12:in `after_approval'
app/models/requests/request.rb:206:in `set_state!'
app/models/requests/request.rb:234:in `approve_by!'
app/models/requests/request.rb:225:in `mark!'
app/controllers/common/requests.rb:40:in `update'
Completed 500 Internal Server Error in 30ms (Views: 9.0ms | ActiveRecord: 9.7ms)