Some pairs of Stack Exchange sites have a certain amount of cross-posting. This is for instance the case for Programmers.SE and Stack Overflow, new users having sometimes a difficulty to understand the difference between those two sites.
Existent users are annoyed by cross-postings and downvote/close the question,
Downvotes are sometimes unkind comments are not a great welcome for users who may become a part of our community, and just didn't know some of the rules here.
Without knowing exactly the internals of Stack Exchange sites, it would appear that it should be technically feasible to prevent cross-posting, for instance:
A user with a reputation inferior to 200 points submits a question on Programmers.SE. Higher reputation indicates that the person is very probably aware of no cross-posting rule.
The site checks if the user has other accounts on other sites (similarly to what is done on Accounts tab of a user profile on stackexchange.com). Most users who cross-post do it with the same profile.
If yes, the site checks if the question was already posted on the other sites by the same user. Most users who cross-post usually have an account on two or three sites, not a few dozen.
If the same question is already posted somewhere else, the submission of the question is stopped and the page is shown to the person explaining:
That cross-posting is not quite welcome here,
That the question is already posted on [Name of the website],
And that if the person considers that the question is off-topic there and on-topic here (with a "feel free to read the FAQ to know what's on-topic" link), the person should flag the question on the other website for migration (with precise instructions).
I think the feature shouldn't have a lot of negative impact in terms of performance/load (but, again, it's only speculation, since I don't know the internals of Stack Exchange) and would decrease the number of cross-posts and increase the user experience for the newcomers.
How about implementing it?