Pesachim 9:10

In the case of two groups of karban pesachim that got mixed up, the groups should make a condition in order to use the karbanos. Each group says if this korban is ours, good, and if not we take one person from the other group and say we are going away from our pesach and going with him on his pesach.

Pesachim 9:11

The same is done if two people each have their own korban pesach and it got mixed up. Each one takes a new person on the korban and makes the same condition as the previous mishna.