Every time I notice close votes due to posting on several sites I wonder how the one who reported that knew this. There are probably some users who are active on several sites.
But today I also wondered as the title says: If a question is posted on several sites exactly the same then it should be trivial for the system to detect such postings automatically. And maybe even delete the later ones automatically.
This has been suggested on the main meta a bunch of times; the original post seems to be Can we automate detection of cross-posting?. There doesn't seem to be any official response, so I don't know if Stack Exchange is opposed to the idea or just hasn't prioritized it.
Most of the time I presume users who are active on more than this site notice the cross-post. This is how I notice cross-posts between SO and U&L (although I'm not even remotely as active there as here).
Sometimes, you can spot copy-paste errors in questions, such as
0
down vote
favorite
where the user has simply copied by dragging across the voting arrows and the favourite button when copying their question from another site. It's then trivial to look for the original question across the network.
