Cross-posting is discouraged in general, because it can lead to duplication of effort by people answering on different sites. However, it is appropriate under some circumstances.
Most of the advice discussed by Joonas Ilmavirta in the context of cross-posting to Math Stack Exchange is applicable in general. To quote Joonas, if you're considering cross-posting, you should do the following:
- Wait several days, not just hours. I would suggest a week.
That is, it is only appropriate to cross-post if you have waited some time after your first post and have not received any answers or have received only unsatisfactory answers. Be patient, and wait for results before cross-posting.
- Provide links between the two versions. The MO question should have link to the one at MSE [or other site] and vice versa. (This is for honesty and avoiding duplicate efforts for those who answer.)
You should be up-front about the fact that you are cross-posting. This helps others to see what progress has been made and provides context for your question.
- Make sure the question is on-topic at the new site.
This applies to all questions, not just cross-posted ones.
- If you have received helpful comments for the old question but no answers, take them into consideration when posting here.
Do not simply copy and paste the old question from one site to another. Revise to to reflect what you have learned so far.
- MO and [other sites such as] MSE require different merits for questions to be well-received. Not only are the scopes different, but also the communities value different kinds of background information.
As a corollary, you should revise your post to meet the norms of the community you are cross-posting it to.
You should not cross-post instantaneously.
If you realized that you asked in the wrong place and there are no answers yet, simply delete the old one and ask in the right place.
However, if you have received comments or answers on your question, it is rude to delete the old question and cross-post elsewhere.
Finally, you should also consider the policies of the other site you're posting to regarding cross-posting. For example, Mathematics Stack Exchange has guidance on cross-posting between their site and MathOverflow.