Are you doing a autonumber on Field1? If yes then you can try the following:
I have not tried it, but what if you create a variable, lets say vCount
LOAD Count(AutoNumber) as AutoNumberCount
LET vCount = Peek('AutoNumberCount')
and then in your incremental load do AutoNumber(Field1) +$(vCount) as AutoNumber, but do this on a resident load table with a where condition specifying Where not Exists(Field1).
Again, I have not tested this, but I feel a variation of this should work for you.