Made article attachment creation thread safe.

lib/import/otrs.rb

@@ -668,6 +668,7 @@ module Import::OTRS
 
           # import article attachments
           article['Attachments'].each { |attachment|
+            begin
               Store.add(
                 object:      'Ticket::Article',
                 o_id:        article_object.id,
@@ -680,6 +681,10 @@ module Import::OTRS
                 },
                 created_by_id: 1,
               )
+            rescue ActiveRecord::RecordNotUnique
+              log "Ticket #{ticket_new[:id]} (article #{article_object.id}, Content-ID #{attachment['ContentID']}) is handled by another thead, skipping."
+              next
+            end
           }
         end
       end