So, from one position, with small modifications, we take more problems. The modifications are : change of the position of one piece, addition or removal of one piece, moving all the pieces by one row or one column, change of the direction of the board, change of the piece on a square, or something similar.
The twins are very common in helpmate problems.
Third Prize, “The Problemist”, 1965
(a) diagram : #2
(b) Twin with Ba5,
(c) Twin with Sa5
The late professor Zappas Byron, (1927 – 2008), was the first Greek problemist who became International Grand Master in composition. The two-mover here has got twins, differing only at the piece on a5. The piece on a5 gives two of the mates. The mates change when this piece changes. The solutions are:
Problem (a) with white Ra5 : Key 1.Rb5! (waiting)
Problem (b) with white Ba5 : Key 1.Bb4! (waiting)
Problem (c) with white Sa5 : Key 1.Ba2! (waiting)
[This post in Greek language].