Why am I getting this message when deleting contacts?

In Lightning Experience, when I try to use the delete button on a contact record page or choose edit from the drop down list on a record in a contact list view, I get this error message: The link you followed wasn’t valid for your session. Return to the previous page, refresh it, and try again.

If I try to delete in Classic, somehow I end up with duplicates that are "created" (somehow, automatically behind the scenes) by other system administrators.