Prolog: Filtering a list?

SWI-Prolog offers exclude/3 and other such meta-predicates. Your original problem can be coded like this:

are_identical(X, Y) :-
    X == Y.

filterList(A, In, Out) :-
    exclude(are_identical(A), In, Out).

Usage example:

?- filterList(A, [A, B, A, C, D, A], Out).
Out = [B, C, D].