Duke Luca Abele's body double looks exactly like the duke in every way. The only way to tell them apart is to listen to them talking and to look for specific behaviours: in particular, the body double is much nicer than the duke and he smokes, whereas the real duke does not. The double can be anywhere inside the Grand Palace, as can the duke, though they are not typically in the same place at the same time.
You can choose to plot against the duke with the double in order to eliminate Luca Abele non-lethally.