Duplicate plug/slot names inside the core snap

[zyga-snapd]

@niemeyer

The first point and the respective PR #2932 looks unrelated to this issue. That’s about validating the names, and all names being discussed are valid from a syntax point of view. The real problem is that they are duplicated, and that’s completely wrong and shouldn’t be allowed by snapd at all during installation. If that wasn’t yet fixed, can we please make sure it is fixed in time for 2.24 (cc @mvo)?

Indeed, I assumed that the validation would detect the duplication. We may need a deeper look.