Maybe let them see each other trough like a fence or something. Here's what some ohther people have said about the subject:
I personally think it would most likely not work. The mother would see it as a intruder and would probably kill it.
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I reintroduced a runner duckling that was a late hatch at 3 days old when its siblings were 7 days old, it was peeping in my hands when I took it out and mom looked at me like why do you have my baby. He ran over to his older and MUCH larger siblings and all was well.
I would maybe take the duckling outside and see how mom is reacting to the peeps. If you could place it with the ducklings so that it isn't obvious that its coming towards mom but part of the group it may be okay. Maybe have someone else distract mom and dad with treats and then put baby with siblings.
I don't think ducks count so I can't see that mom would be like I only had __ now theres one more.
If you try, just stand by in case of a problem. With mine there was an obvious size difference, the older ones were 2X the size and the little one was fine.
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Hope this helps!