Each character slot has a respective list, you cannot duplicate anything on that list that is claimed, for that character slot.
Two people cannot reserve the same main pokemon.
Two people cannot reserve the same secondary pokemon.
A person can reserve a main pokemon, and then a secondary that is claimed as a main and still open on secondary.
It's a bit confusing when put into words like this, but
the simple answer is no for your first character. Just make sure you are on the correct
Reservation List so you don't get confused.