Make account endorsements idempotent (fix #19045) (#20118)

* Make account endorsements idempotent (fix #19045)

* Accept suggestion to use exists? instead of find_by + nil check

Co-authored-by: Yamagishi Kazutoshi <ykzts@desire.sh>

* fix logic (unless, not if)

* switch to using `find_or_create_by!`

Co-authored-by: Yamagishi Kazutoshi <ykzts@desire.sh>

app/controllers/api/v1/accounts/pins_controller.rb

@@ -8,7 +8,7 @@ class Api::V1::Accounts::PinsController < Api::BaseController
   before_action :set_account
 
   def create
-    AccountPin.create!(account: current_account, target_account: @account)
+    AccountPin.find_or_create_by!(account: current_account, target_account: @account)
     render json: @account, serializer: REST::RelationshipSerializer, relationships: relationships_presenter
   end