I only have one duck that is laying at the moment and the one on the left looks like the first one she laid and the one on the right looks like the fifth one she laid. She is new at it though and is till getting the hang of the laying thing so I chalked it up to that. It is possible that it is a different duck but it's also likely that they belong to the same duck. Is there any reason in particular that you are worried bout who laid it? if it is a big issue you can separate your females for one night since they lay early wait until after they have laid to let them go play together and you will be able to tell if it was laid by a different duck.