So are you saying you go in and the invoice is defaulted to be the dmax, say invoice 100, then someone else goes in and gets invoice 100 at the same time and you have duplicates?

If so, I would instead assign the invoice number with the dmax on a "save" button at the end of creating the invoice. Then you know you're always getting the most current record and if other people are saving invoices at the same time it will be the most current one. Otherwise you need to instantly "save" that record when you open the form to lock that invoice number into the table, which if you end up not needing to do the invoice or something will result in a blank record with an invoice number if you don't delete it.

I have a lastnums table that i use for receipt and invoice numbers. Once data is captured on the form, the creation of the invoice is done in VBA and written to my tbl_GroupBilling table. I have a "Create Invoice" button on the form that triggers that code, which does some checks and validation and some user message acknowledgments before an invoice number is assigned. After the data is written to the table, the report (invoice) is triggered.

So you're going through these checks, then it automatically assigns an invoice, then are you saving the record at this point or is the record still dirty?

If the record is created and then saved at the point of assigning, it should update to the table instantly and the next person to push the button would see that it exists and grab the following number.