Adding (already existing) product to catalog with XML

Sudha,
I have a nightly product feed. As I don't know which products already
exist 
in my catalog, I have to add an include rule to every (new or updated) 
product if I use virtual catalogs.

My tests have shown that one cannot include a product once and then
include 
it again later. Am I correct or is there something I'm missing?

Day 1:
<VirtualProduct ProductId = "1" .... listprice = "12.13"/>
<IncludeRule baseCatalog="X" ProductId ="1" />

Day 2:
<VirtualProduct ProductId = "1" .... listprice = "11.13"/> 
<IncludeRule baseCatalog="X" ProductId ="1" />

This is the same product. the second day's include rule throws an error.

Is there any way I can get past this save having to read the entire
catalog 
with the API before adding include rules?

Thanks,
step.