Complement1[l1, l2] where l1 and
l2 are lists returns a list of elements from
l1 not inl2. Multiple occurrences of an
element in l1 are kept and multiple occurrences of an
element in l2 are dropped if present in
l1.
Complement[{a, b, c, d, e, f, e}, {a, b, c, d}]\{e,f\}
Complement1[{a, b, c, d, e, f, e}, {a, b, c, d}]\{e,f,e\}