New empty cells are allocated for any unbound variable in the original:

?-
G=f(X),duplicate_term(G,H),X=2.

G = f(2),
X = 2,
H = f(_6402).

Can deal with cyclic structures:

?-
A=[1,K,3|A],B=[a,b|A],cyclic_term(A),
duplicate_term(B,BB),nth0(3,BB,foo).

A = [1, K, 3|A],
B = [a, b|A],               % B = [a,b,1,K,3,1,K,3,1,K,3,....]
BB = [a, b|_S1], % where    % BB as B but with the copy of K bound to 'foo'
    _S1 = [1, foo, 3|_S1].