Potential confusion with errors when adding duplicate objects

[clenk]

TAXII 2.1 Section 5.5 Add Objects says:

If a client publishes an exact duplicate of an Object already present in the Collection, the server MUST not return an error for that Object.

The way it's worded may confuse some readers because the error message type in 3.6.1 is an error for an entire request rather than for a single object. The cti-taxii-server library interprets this line to mean don't return an error as defined in 3.6.1 but it might be interpreted to mean that the status-details returned for that object can't contain an error/failure.

Suggest changing to "not return an error for that request." Not urgent.

Originally brought up by oasis-open/cti-taxii-server#131.