Looks like that repository in your sources list uses the Signed-By option multiple times, so apt doesn't know which key to use to check the signature. Or maybe it's a related problem, but there seems to be a case of conflicting keys.

When we encounter the error, it’s usually because apt has found multiple conflicting instructions regarding the GPG key to verify a particular repository.

Specifically, this conflict arises from duplicated repository entries, incorrect or outdated GPG keys, or improperly configured repository settings.

I think currentshaft is on track, I'll go away now...Good Luck