Add a uniqueness constraint to `MergeRequestsClosingIssues`

app/models/merge_requests_closing_issues.rb

 class MergeRequestsClosingIssues < ActiveRecord::Base
   belongs_to :merge_request
   belongs_to :issue
+
+  validates_uniqueness_of :merge_request_id, scope: :issue_id
+
+  validates_presence_of :merge_request
+  validates_presence_of :issue
 end