In addition to what Michel said, the reason for this warning is that 1-1 uid relationships are a new feature (which uses the syntax of what was previously a 1 to many relationship) and we didn’t want users to overwrite their data by mistake. For now you’ll have to update this triples by deleting and adding.
We might remove the warning in a future release once most users are aware of the distinction between predicates of type uid and [uid].