You could add a short cooldown timer so that only the first tap is registered.
Add the Timer behaviour to your object.
Add a number Instance variable to your object called "Firing" set to 0
"Instance.Firing = 0"
____Start Timer for 0.2 seconds tag "Cooldown"
____Set instance.Firing to 1
____Call Function "Fire!"
On Timer "Cooldown"
____Set instance.Firing to 0
This will stop your player tapping twice in quick succession (such as a two-fingered tap) as the first tap immediately sets Instance.Firing to 1. However, the biggest limitation here is how fast you allow your player to shoot. You can adjust the timer to accommodate this.