No, it can't hurt anything. My memory of the Duet is that it has a pair of jack sockets on the breakout cable that can be used as the audio output to powered monitors. However, I believe they are at -10dBV level (i.e. domestic level), and unbalanced, as you say. I have no information on whether the jack sockets are TRS with the ring actually grounded or just TS with the potential of the ring connection left floating.

Anyway, if your monitors have enough gain on the jack inputs to deal with -10dBV levels, then the safe thing to do is to use TS jack leads, when the ring contact at the monitor end will automatically get grounded. If you are prepared to open up the Duet break-out cable and examine the jack sockets, or buzz them out using a TRS plug and a multimeter, you may find that they are actually TRS sockets with the ring already grounded. If this is the case, then you could use TRS jack leads to the monitors. You will notice no sonic difference.