If a group lost their Offering, and appointed someone to find it and slaughter it, and in the meantime they slaughtered another Pesach Offering: Pesachim 98b
If an individual told a group to slaughter for him if he doesn't show up, and in the meantime he slaughtered for himself: Pesachim 98b
Where two groups/individuals designated each other, and both slaughtered for each other: Pesachim 98b
Where two groups/individuals didn't designate each other, and both slaughtered for each other: Pesachim 98b
Bringing a korban pesach as a korban shelamim if it was lost before Pesach and its owner used a different animal as the korban, and it was found after the time for its Pesach use: Zevachim 7b, 9a-b, 11a-b; Temurah 19a, 20a
