I am used to Stack Overflow, where when somebody asks the same question on Stack Overflow, and Drupal Answers, I flag the question for moderation attention (using "other" as flagging reason), explaining the same user asked the same question on Drupal Answers, and giving a link to the question on that site. What the moderators do in these cases is closing the question on Stack Overflow, and eventually comment on the question being cross-posted; sometimes, the moderators add a note for the OP about not cross-posting questions that are on-topic on more than one site.
Notice that in the case I flag the questions, the questions are carbon copies, not questions that are rephrased to ask slightly different, or follow-up questions; they are not even questions that are tailored for the site where they are asked.
Should I do the same for questions that are asked on EL&U, and then are asked on ELL when somebody suggest the other site for the question?