To follow up on this a bit more since it was raised in an FAQ discussion thread…
I believe it's a bug for that topic. The Solved plugin which is providing the solution-marking ability is supposed to allow only a single solution to be marked.
The person who started the topic can change their choice as many times as they want. Marking a different reply as the solution un-marks the previously chosen reply. The notifications go out whenever a solution is marked, so if the OP tried marking more than one, you'll get more than one notification. Discourse can be rather chatty with notifications .
It's OK not to know how things work when you're new! You might try poking around on our FAQ page, and in the FAQs category to get a handle on how we do things around here. And please always feel free to post here in meta if you have a question about how this site operates .