Can digits repeat in the sandwich?

The sandwich is simply a stretch of consecutive cells within one row or one column. Standard Sudoku already forbids any digit from appearing twice in a line, so the sandwiched digits are automatically distinct. Killer Sudoku needs an explicit extra rule because its cages can bend across rows, columns, and boxes; a sandwich never leaves its line, so the classic rule does all the work. A two-cell sandwich summing to 8 can be {2,6} or {3,5}, but never {4,4}.

There is a second, easy-to-forget restriction: the 1 and the 9 are the boundaries of the sandwich, and each appears only once per line - So neither can ever be inside it. The filling is always a subset of {2,3,4,5,6,7,8}. This shrinks the combination space dramatically. A three-cell sandwich summing to 12 looks like it might allow {1,2,9} - It does not. The only valid sets are {2,4,6} and {3,4,5} (and {2,3,7}), all drawn from 2-8. Always enumerate combinations from the seven middle digits, never from all nine.

Distinctness is what makes forced combinations exist at all. Because no digit can be reused, extreme sums have very few realisations - Two cells summing to 5 must be {2,3}, three cells summing to 21 must be {6,7,8}. Each forced set behaves like a naked tuple: its digits can be eliminated from every other cell in the line, and from any box that fully contains the sandwich segment. Uniqueness turns an arithmetic clue into hard placement logic.