Like her pal Archimago, Duessa is primarily known for never being what she seems. In contrast to Una, her very name implies she is two things, not one ('duo' meaning two) and so it makes sense that she is always trying to find ways to deceive people that she meets.
Her skills of disguise are obviously quite impressive since she can pretend to be Fidessa, whose name itself ('faith'), means exactly the opposite of what Duessa stands for. Indeed, Duessa is so opposed to the ideal of true faith, embodied in Una, that when she becomes the girlfriend of Orgoglio and rides her pet monster, she is described as looking exactly like the infamous Whore of Babylon who is a kind of Antichrist in the Book of Revelations.
Finally, we can see Duessa as a manifestation of the poem's larger obsession with duality (see Symbols, Allegory, Images) and the dangers of false or misleading replication. Duessa's bad. You don't want to be like Duessa, even if being Duessa means getting to ride around on a pet monster. It just sounds fun.