Currently, uniqueness of tracestate keys is not defined anywhere. The closest it gets is the section on updating a key. However, there's nothing that says a key can't be added twice, at least not that I can find.

Code written poorly could easily merge together 2 tracestate headers and result in duplicate values. Consider messaging where you have a "current trace" and you are reading incoming state also.

This should be addressed with a MUST somewhere, ex MUST not duplicate keys or MUST ignore duplicate keys, reading the first.