There really isn't a good reason to check every single timer you use to see if it's in use. In addition, some timers WILL be in use by you and you want to overwrite them. So a check to see if it's in use wouldn't help in the first place in those situations.
Yes, you *might* overwrite a named timer accidentally, but the chances are so low as to be non-existent as long as you're not naming it something like mytimer or countdown or some other name that has a decent chance of being used by someone else (not as much as a numerical timer, but it's still high enough to avoid). As long as you name it something unique that isn't likely to be used by anyone else, then you don't need to check if it's in use. That's a waste of time and effort and in some cases, as mentioned, it's not possible to easily tell the difference if you want to overwrite your own timer but not an auto-generated timer.
