Reststream Twitch stream?

What if someone restreamed my stream while I'm currently streaming live? Can he do that?

Restream can only be used if you have the Streamkey.
And if you are an affiliate, it is forbidden anyway & leads to a permaban on Twitch or exclusion from the affiliate program.

Thank you for the quick answer I have been struggling with one of these for a long time

What exactly are you struggling with?

A small streamer watches me while I'm streaming and comments on my stream in his stream, I don't mind but it's iwie weird has happened several times now. I myself have a small streamer with less than 20 viewers

Less than 20 viewers doesn't sound SO small anymore.
It's not bad I would say.
Would just report him on Twitch if it annoys you, or even better send him a Whisper message beforehand that he should please stop.
I'm not sure whether this is legal or illegal, but I suspect that it is not at all desirable as soon as the person concerned has a problem with it or does not give the other permission.
After all, that's your content. By the way, if he should talk badly about you, that's very much illegal.

So 20 streamers or let's say more than 10 is by no means bad, on the contrary. This is a small base that, as you continue to work on it, grows and becomes more successful. Whereby the question is also the audience come because you are streaming or through the other streamer?

Why do Reaction's with other streamers who are currently live? Well, unfortunately I can't answer.

Otherwise you can agree to DaDarvid.