Hector, checked my code with the example that you provided but I am still
getting the same error message.  My QWK downlink (also running WINServer
and who is also a hub for a QWK network I am on) also has the same issue. 
Our message posting programs post fine locally to our boards but when they
get uploaded in a REP packet, WINServer's QWK mail processor rejects the
messages as duplicates.  Again, this did NOT happen until the most recent
AUP and the same issue can be replicated on both boards running the same
AUP release of the WINServer software.  Also, this did NOT happen with the
release before the most currentt AUP update. 
--- Platinum Xpress/Win/WINServer v3.0pr5