How do you deal with it ? Specially if you know that person haha !
I won't disclose the name of the artist or show their artworks because I don't want to start any drama, I just want to hear your experience and share mine xD
I know it'll be hard for you to judge the legitimacy of my claim but I talked about it with a couple friends and they thought the same.
So basically last year with a group of artists I started the idea of creating new OC within a shared universe. The artist was in the group (we are a part of the same artist community so we know what people are doing, what's their projects etc)
We went on creating our new OC but when I saw the one from the artist, I felt weird because the character looked pretty similar to one of my main character.
I know she looks pretty regular

but trust me the other one have similar features, like the long blue-ish jacket with the sleeve stopping halfway, the haircut that can feel similar, the redhair, the eye colors, the black choker.
The other character have a black shirt and a grey short which is different from mine but it's basically an inverted color pattern, the black slim pants and the midtone long shirt.
But then it didn't stop there, the story of my characters is set in france and while the artist's characters seems to be apart of a fictional setting (but still set on earth), the new oc lives also in france.
So even though my character is quite regular, I find it a bit odd to even create a character that could look similar to one of your friend/acquaintance's creation.
Also within the same community one of my friend had a similar experience with 2-3 of his OCs which kinda made us believe that some people didn't know the difference between taking influence and straight up copying xD
Just for the info, they are decently experienced (some started to be pro and other are in their last year of art schools) so they know better.
Sorry for the long post 
EDIT : Since then I reworked the design of my character, they are still quite similar but I made the design more personal.