I have not tried this but not knowing any better here is how I would approch it.

I am assuming that the feild that you do not want to have duplicate values in is your unique identifier for each record. If this is the case you want to have Access AutoGenerate the number for that field when it is created. You set this up when you set up that field in the Table.

Now when you want to add a field through automation. Have access create the record then fill in the other data. You should not have to worry about that field