Is asking the same question as in math.SE in sci.math, forums, etc. considered spamming?
It is perfectly acceptable to ask a question on Stack Exchange and also on other forums outside the SE network. Barring issues of copyright, we are largely agnostic to what exists elsewhere in terms of our own quality standards. So, as long as the question is up to our quality standards, it is okay to post here.
There's certainly no requirement to do so, but I think it's usually a good idea to include links between wherever you post the same question. There's probably some overlap between such forums, so that can help avoid duplicated effort. It also helps M.SE users see if there's an answer or helpful discussion elsewhere (which can help them provide an answer here) or users of other forums see that there's an M.SE question, and perhaps write an answer here.
It's also worth noting that we have a narrow definition of spam: spam is limited to posts that promote a product rather than ask/answer a mathematical question. However, I assume that intent of the question was more about whether this is good form than whether it's "spam."