Can digits repeat within a cage?

Killer Sudoku cages always sit within rows, columns, and boxes. When cage cells share the same row, column, or box the standard Sudoku no-repeat rule already prevents repetition. But even when cage cells span multiple rows, columns, and boxes, the cage itself carries an independent no-repeat constraint.

This is an inherent rule of Killer Sudoku - It is always in effect on every cage, regardless of size or position.

Without the no-repeat rule, a 3-cell cage summing to 9 could include dozens of digit combinations including {3,3,3}. With the rule, only {1,2,6}, {1,3,5}, and {2,3,4} are valid - Three combinations instead of many more. This restriction is exactly what makes forced combinations possible: a 2-cell cage of 3 must be {1,2} because no other unique pair of digits 1-9 sums to 3.

Every time you confirm a digit in one cage cell, that digit is eliminated as a candidate from all other cells in the same cage - Even if they are in different rows, columns, and boxes where the digit would otherwise be allowed by standard Sudoku rules. This is an additional elimination source unique to Killer Sudoku.