twice([], []).
twice([Head|Tail], [Head, Head|Tail2]) :- twice(Tail, Tail2).
test_answer :-
twice([a, b, c, d], L),
writeln(L).
test_answer2 :-
twice(L, [1, 1, 2, 2, 3, 3]),
writeln(L).
test_answer3 :-
twice([], []),
writeln(‘OK’).
test_answer4 :-
twice(L1, L2),
writeln(‘OK’).
test_answer5 :-
\+ twice(L, [a, a, b]),
writeln(‘OK’).