How can i create a number field that auto populates and does not duplicate sequential numbers?

This is for an auction and each auction items requires a unique number.

I would suggest you use a Unique ID widget in your form.

The explanation for each property (Current Index, Prefix, and Number Padding) is found in this guide: How-to-Add-Custom-Unique-IDs-to-your-Form-Submissions

I hope this helps. Feel free to contact us again anytime if you need any further assistance.

Thank you.