Thanks for your reply.

I've clicked Log Out, and then Log In and it says that I'm authenticated as my username.

However mousing over any item in the game still shows "Last User: " with another user's name.

At this point should I uninstall Factorio from steam entirely?

Thanks for any help.

My factorio game (and DLC) are bound to my steam account and I've been playing for a while now.

However ever since playing on a friend's server my username seems to have changed to HIS username. It makes no sense at all. My steam name and my factorio name are not reflected in the games UI elements ...