I have never seen this!
Converters make some sense, but I find it hard to believe that many duo owners would have one.
I think every person I've ever known with a Duo has had a converter. Even I have one. Everyone I know bought a Turbo Duo because of all the cool games featured in EGM and such...games that ended up never coming to the US, so importing eventually ensued. Importing is expensive, but the price of PC Engine games was cheaper than import SFC games by $40 or more, and if you could afford a Duo (three times the price of a SNES) then you could probably afford import games for it. Furthermore, even though people pay shit-loads of money for these converters now, they were often free with a game in the early 90s. They are just cheap pieces of crap after all.
As for the door; removing it won't allow the TV Game Computer Cartridge Converter (aka: Barney) to fit. It has to be physically hacked to pieces. It fits in an Express, PCE core (all three), or a TG-16 no problem. The door does get knocked off pretty easily, but it also snaps back into place pretty easily providing you didn't snap it.