AP-123 - Forgive me for suggesting something so simple, but try hitting your Insert key on your keyboard.
It sounds like it has been "triggered" so that as you edit you're inserting. Hit the button again and it should change to a more traditional/expected edit behavior.
It sounds like it has been "triggered" so that as you edit you're inserting. Hit the button again and it should change to a more traditional/expected edit behavior.