Why does this function prompt twice?

I’ve written a bookmarklet for clicking a controllable amount of times on a HTML element (for clicker games, the like). However, when I run it, it prompts “How many times do you want to click?” twice.

To the best of my knowledge, the second prompt doesn’t do anything. Is there a way I can get rid of it?

Code:

javascript:
var clicked = false
document.onclick= function(event) {  
    if (!clicked) {
      var target= 'target' in event? event.target : event.srcElement;

      var clickamount= parseInt(prompt("Enter time to click "+target.id))-1;

      for (let i = 0; i < clickamount; i++) {
          target.click();
      }
      clicked = true
    }
    
};