Finding all the unique permutations of a string without generating duplicates

Question

Finding all the permutations of a string is by a well known Steinhaus–Johnson–Trotter algorithm. But if the string contains the repeated characters such as
AABB,
the

Answer 1

I think this problem is essentially the problem of generating multiset permutations. this paper seems to be relevant: J. F. Korsh P. S. LaFollette. Loopless array generation of multiset permutations. The Computer Journal, 47(5):612–621, 2004.

From the abstract: This paper presents a loopless algorithm to generate all permutations of a multiset. Each is obtained from its predecessor by making one transposition. It differs from previous such algorithms by using an array for the permutations but requiring storage only linear in its length.