Hello there, @shoot-creedmoors.

Thanks for joining the Community. In QuickBooks, there’s an option to warn you when entering a duplicate bill number. 

To do You'll want to make sure that the feature is turned on. Here's how:

1. Click the Gear icon, then go to Account and Settings.
2. Select Advanced from the left menu.
3. Go to the Other preferences section.
4. Then, turn on the Warn me when I enter a bill number that’s already been used for that vendor option.
5. Click Save, then Done.

You'll now get a prompt that the bill number you're entering is a duplicate.

You can also visit our QuickBooks Community help website to get more tips and related articles to your future concerns. 

Feel easy to visit us again should you have more concerns. I'll be around to guide you again.