update quantity of same same upc

barcode - preprocessing/vba/combination

*disclaimer: I have a limited (2-3 projects) experience with barcodes, and have only used Symbol so far

Simplest first: formatting -- the barcode reader should allow for some processing, e.g. if your upc is 027434011297, you should be able to program it to send to Excel upon a successful scan:
- a single quote ('), then the value scanned (027434011297) resulting in a label (vs. a numeric value)
- other combinations transforming the upc (e.g. 027-434-0011_297), including conditional logic based on characters present within the scanned data)

As far as how to decide on incrementing - based on the scenario you present, I would probably use a VBA/dialog which would accept the scanned UPC value into a textbox, then hit an accelerator key (tied to the VBA code, e.g., Alt-U for Update_Sheet or whatever you like). This logic would then do your bidding:
- if UPC value exists in list, increment it (or prompt to increment it)
else
- add UPC value to list and start its quantity at 1 (or prompt accordingly)

Again, the scanner can be programmed to send the upc value itself, then pause, then send a keystroke mapped to the accelerator on your form, or Tab/Enter, or whatever you can imagine.

If you want to get fancy and set time thresholds, you could certainly catch the date/time of the scan and make decisions based on the date/time of the last time the same UPC was scanned, recorded on the worksheet.

The key is understanding the scanner capabilities and combining with the the Excel automation you prefer.

Hope this helped.