Doppelgänger village help

Hi guys, this is my first post here, loving the podcast.



Am looking for some help with my village.



My game is in the sword coast and is 5E.



Basically 4 doppelgängers are using the town as a breading village impregnating the women, but they are being very sneaky about it, waiting until the husbands are out and changing into them to sleep with their wife’s or some other means.



Am looking for ideas of clues I can drop to my players about what’s going on.



The whole village is a small demi-plane created by a wizard to test the party and find out if they are smart enough to go on the quest he has for them.

A little background about the village



The village of Rose Tower used to be a small mining village but the mines have dried up so most of the people have moved away from the village.



When my player arrive in town they notice a large number of children running around, and a couple of women are pregnant. For a village of this size there shouldn’t be so many children.




Thanks to the Block for the basic idea, 


Thanks in advance!!!