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Singular:
the duck’s feet

Plural:
the ducks’ feet

This does not change when the object ends in an S. It is not, for example:

Erin Evans’ witty sentence
Erin Evan’s witty sentence
Erin Evanses witty sentence (I wish I’d never seen that one, but yes, some people have tried it).
Erin Evans witty sentence (Ditto)

People may insist otherwise. They are liars. It’s in the CMS.

And now you know!*

Evans’s: You don’t have to like it, but that is the proper usage.

*How many of you said,  “And knowing is half the battle!” ? I’m betting…seven.