Generally speaking, the SMB server (Nucleus) only advertises the shares on the network and it’s the client (your Mac) that performs the mounting into its own filesystem. And I think this is what happens here, the Nucleus is the passive part. So you need to figure out why your Mac is mounting the SMB share twice.
It certainly happened before for other people, googling “SMB share mounted twice Mac” finds many instances, e.g.
https://www.reddit.com/r/macsysadmin/comments/dsw68s/smb_share_win_2019_server_shows_twice_on_mac/
It actually probably doesn’t matter. File servers are designed for sharing files to different clients at the same time, which would have them mounted. Usually it would be different machines mounting the shares, it’s just that in your case it’s the same machine apparently doing it twice.
I don’t think that TM backs up network-shared mounted folders, so there’s probably no need to exclude it. I certainly didn’t exclude it from my backups and it never occurred to me, but I am not home and can’t verify. I’m pretty sure though (and nobody on Google seems to be talking about doing that)