Labels could be duped according to WIT interface

[sporkmonger]

Currently, the WIT format doesn't support map types. We're instead using a list<label> type, essentially a list<tuple<string, string>>. While the SDK prevents dupes by working with the HashMap type, this does not prevent dupes for anything that's using the WIT interface directly. Dupes from a single plugin (as opposed to collisions across plugins) should probably be considered an error condition.