when Im building the duo decks, can i make decks like this: , , etc? basically just using an element more than once
In the case of pawns, sure; the 5 remaining elements which are left after choosing the marks can be used to make monos, duos, trios, or even quint decks if you'd like.
For the main pieces (2 rooks, 2 bishops, 2 knights), you have to divide the 12 elements into 6 duo decks, so you can't use
for your bishop1, and then
for your knight2. They don't have any restrictions on their mark, though.