There is much overlap between the SO, SF and SU sites. For instance, questions about shell programming are programming related (since they are about programming), about system administration (because they manage the shell) and for single users too, if they want to script their computer.
I suggest allowing questions that overlap sites to be posted to more than one site simultaneously in order to:
- avoid duplication between sites
- get answers from all 3 communities.
I suggest that the answers be visible on all relevant sites, and that any votes apply to all relevant sites.
Edit: For instance, questions about VIM have appeared in SO (637/222780 questions) and in SF (17/8142 questions). VIM is a valuable tool for programmers as well as sysadmins. Where do such questions belong? I didn't review all the questions in SO but the ones in SF could easily be in SO as well.
How about Perl? 9/8142 on SF and 1772/222780 on SO. If you're a sysadmin but not a programmer, Perl is most certainly relevant.
These are two cases where there is overlap. I'm sure there are many more.
Edit: Here are some good examples of questions on SU that are relevant to SF and SO as well:
Remove lines efficiently in Vim
Edit: Here's another example of such concerns: