Two identical timers on iPhone

When you set a timer it does not delete previous timers you have set, so if you previously set a 30 minute timer that has expired, then set a new timer for 30 minutes you will have 2 30 minute timers, on current and one historical. This is a relatively new feature so you can reuse timers that you have used previously, without having to create a new timer. If you open the Clock app and tap on Timers you will see all of your recent timers.

How are you setting the timer?

Yes, it does, because it is a recent timer. It isn’t running, it is just a placeholder. When your new timer completes it will disappear, and leave just the recent timer.