Luckily pretty much all of the characters I have are original enough not to get compared to an existing canon character, so I've never had to deal with a lot of it. There was one time a friend of mine said that one of my Wings of Fire oc's looked like a character that exists in the book. I looked said character up, and other then just having the same/similar color scheme (it can be kind of hard to make up a unique color scheme for a WoF oc since color combinations depend on the tribe, so there's a limited range you can only work with and no doubt you're bound to run into another character with a color scheme similiar/exact to yours), the character had their own physical traits that the canon character didn't have. Was really a minor thing that didn't bother me, nd they took their claim back when I compared two images and pointed out the differences.
I can definitely imagine it would irritate/annoy most people though to have characters of theirs compared to a canon character, especially multiple times. And I feel bad for users who have to deal with it, especially if it's traits that are used a lot in character designs. Then again, I also have a pretty laidback/calm attitude so if any of my characters were somehow compared, it wouldn't really be much of a bother and I would just either ignore or politely correct the accuser.