Has your Duo stop loading any CD games or audio
discs? Open the CD lid/cover and see if the CD
lens/laser is parked all the way to the right.

Normally, when you turn on the Duo and try to run a
CD game or audio disc, the laser unit will seek for
a readable track. When none are found, the laser
unit will park all the way to the left.   

For some reason when playing an incorrectly burned
or incompatible CD-R brand/type, the Duo's laser can
get stuck all the way to the right. I've run into
this problem on 3 different Duo units. Though I
haven't seen this happen to the add-on CD units as
of yet.

The fix requires you to open up the Duo and manully
rotate the motor that drives the CD lens unit on the
rails. This is very easy and only requires taking
the Duo top cover off. Number 1 is the gear you need
to rotate until the CD lens unit is parked to the
left - re-attach the cover.

If you use the same bad/incompatible media, the lens
unit will get stuck to the right again. Note: the
lens unit usually doesn't get stuck right away from
a bad/incompatible CDR - it can happen at random.
Use TocFixer to very CD image is read for burning
and stick with known compatible media(Verbatim).