FIX: Adding a user to a group twice under concurrency

This prevents an error from being raised / logged.

app/controllers/groups_controller.rb

@@ -321,8 +321,14 @@ class GroupsController < ApplicationController
       ))
     else
       users.each do |user|
-        group.add(user)
-        GroupActionLogger.new(current_user, group).log_add_user_to_group(user)
+        begin
+          group.add(user)
+          GroupActionLogger.new(current_user, group).log_add_user_to_group(user)
+        rescue ActiveRecord::RecordNotUnique
+          # Under concurrency, we might attempt to insert two records quickly and hit a DB
+          # constraint. In this case we can safely ignore the error and act as if the user
+          # was added to the group.
+        end
       end
 
       render json: success_json.merge!(