Yes, it is a problem: the same question shouldn't be posted to multiple SE sites because it tends to result in duplication of effort and different answerers missing each others' contributions. If we find that a question has been cross-posted, we'll consolidate it on one of the sites where it is on topic.
If you have a question that is on topic on more than one site, there are a couple things you can do:
- Split it into two questions; ask the computational part on the computational science site and the physics part on this site. Most questions that you think fit on multiple sites actually don't, because they're too broad and should be split up this way. The communities of related sites usually coordinate to make sure their scope are split in this way, i.e. so that there's very little overlap. (Usually; not always.)
- If the question really is on topic on both of the sites, choose one or the other to post it on. Make sure to adapt the question to the audience of that site - for example, if you're asking a computational physics question here, make sure it reads like a physics question.
The facilities for handling questions that cross multiple sites' scopes are not the best. People have raised this issue often on Meta SE, so I'd suggest checking there if you want to support better cross-posting or multi-posting features.
