It might not have been right exactly when the duo debuted, but it was around the time tzd took over to sell tti's remaining stock. I think Fulci is correct about the trade in process. If you had any issues with your tg-16 and you called about it (I believe we were having issues with our turbo tap, go figure) they would try to talk you into trading it in towards a brand new duo retail package.
I don't think this was officially or expressly stated on their website ever, nor in their catalogs, but rather a way for them to move slow selling duos via their repair services. Besides, now that they sold you a new duo, you'll want to buy all those nifty new cd-rom and supercd-rom games. See, profit!
We never even sent them the faulty tap btw.
When the duo was released, no one in our area sold much, if any, duo/tg-16 stuff anymore. We had to rely solely on mail-order tzd for our turbo goods. We got Splatterhouse too. It was kinda ironic that we already owned Splatterhouse, but wanted a copy of the purportedly more common pack in, Ninja Spirit. We could not find it for sale anywhere after renting it from a mom and pop place twice years prior. In fact that copy is still sealed.
Oh and for a blast from the past check out tzd via the wayback machine on archive.org. Trust me, it's great! It goes all the way back to 1996...