AO3-6714 Fix duplicate tag set ownership on create

app/controllers/owned_tag_sets_controller.rb

@@ -144,7 +144,17 @@ def new
 
   def create
     @tag_set = OwnedTagSet.new(owned_tag_set_params)
-    @tag_set.add_owner(current_user.default_pseud)
+
+    # Check if the current user already owns the tag set, and only add them if they don't.
+    #
+    # This happens when the user adds themselves explicitly as an owner.
+    # This check is done by direct Enumerable.find rather than ActiveRecord `find_by`
+    # as we cannot use `find_by` before the record is stored in the database with `.save`
+    current_user_ownership = @tag_set.tag_set_ownerships.find do |ownership|
+      ownership.owner == true && ownership.pseud_id == current_user.default_pseud.id
+    end
+    @tag_set.add_owner(current_user.default_pseud) unless current_user_ownership
+
     if @tag_set.save
       flash[:notice] = ts('Tag Set was successfully created.')
       redirect_to tag_set_path(@tag_set)

features/tag_sets/tag_set.feature

@@ -21,11 +21,11 @@ Feature: Creating and editing tag sets
   Then I should see a create confirmation message
     And Maintainers should be "anotherowner tagsetter"
 
-  Scenario: A user should not be able to remove themselves as an owner (AO3-6714)
+  Scenario: A user should not be able to dupicate or remove themselves as an owner (AO3-6714)
   Given I am logged in as "tagsetter"
     And I set up the tag set "Duplicate Ownership" with owners "tagsetter" and the freeform tags "Clones"
   Then I should see a create confirmation message
-    And I should see "tagsetter tagsetter" within ".meta"
+    And I should see "tagsetter" within ".meta"
   When I go to the "Duplicate Ownership" tag set edit page
     And I toggle the owners "tagsetter"
     And I submit