Define a duplist predicate so that duplist(X,Y) says that X is a list and Y is the same list, but with each element of X repeated twice in a row. For example, if X is [1,3,2], Y should be [1,1,3,3,2,2]. If X is [], Y should be []. Check that your predicate works in both directions - that is, check that it works on queries like dupList(X, [1,1,3,3,2,2]) as well as on queries like dupList([1,3,2],Y).