I've had this experience of multiple notifications. But then only the final selection remained. That's why I was surprised and asked this question.
Though quite old (more than two years), and hence possibly outdated, but this thread suggests Discourse doesn't support multiple answers:
If a separate plugin (or whatever be the proper terminology) is applied on our community, that'll be great.
I've seen that for many of the threads, there are more than one excellent answers (quite different approaches), but the OP had to choose one of them just because there's no option to select more than one. Also, sometimes people tend to ask related questions later in the thread, and someone else but the person who answered the original question provides the answer for that and OP selects that post, the original solution marking is overridden and that's pretty unfair (in my opinion, at least) for him/her.