I have added eggs one day after I added the first ones before with no issues, but they were all chicken eggs. If you do it too many days apart, depending on how your incubator is, you'll potentially run into issues with turning them (if you have an automatic turner) if you add them days later (while first set is on lockdown and others still need turned), and raising the humidity for the first set to begin hatching when it should still be low for the second set. I know it can still be done, however. It is just trickier and riskier, I've heard. Also, I am not sure if some being chicken eggs and some being duck eggs would affect it at all either. I have only ever worked with chicken eggs.