I think there's value in having similar questions posted to multiple sites, so long as they're actually on-topic for the sites they're posted to.
After all, I'd certainly expect to see different solutions from a set of Sys Admins than I would from programmers... And if I'm a Sys Admin, I probably don't care to read detailed suggestions for writing my own tools so much as I want a good description of how to use the tools I already have - so there's value in separating these answers as well.
If a question is off-topic for a given site, then it should be closed - and optionally migrated to the site it's on-topic for, where it can be closed again as a duplicate.
Otherwise, let users cross-link as they see fit; after all, it may make more sense to link to answers on another site rather than blindly linking together the questions themselves.