Is there a way to have a validation check to custom fields on invoices. Like duplicate number/value alerts?

Welcome to the Community, @asaalogistics-gm.

Let me share some clarification about the custom fields when creating invoices in QuickBooks.

Currently, an option or feature to detect duplicate figures from the custom fields is unavailable. For now, the system can only identify the same value to the default number field.

I appreciate your input about having this feature available in QuickBooks Online. We make improvements to the product from time to time, and this preference might be available soon.

Feel free to visit our QuickBooks Blog while we assess this to stay updated with all the changes. Also, I've attached an article you can use to learn more about how to customise invoices in QuickBooks: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Drop me a comment below if you have any other questions. I'll be more than happy to help. Wishing you a good one.