You probably wouldn't use a PUT for anything like this, true. But if you're going to mark a message as "seen" in a way that would impact a UI widget like an "unread notifications" red dot, then you almost certainly want to make sure that the state-changing request for that message is a POST, not a GET.

There are just so many ways for GET requests to be delivered to a server (or load balancer, or IP, or domain, or...) multiple times for a given client request. That capability is built in to HTTP and exploited in more places than you can ever hope to account for, or even detect.