What makes an ACH account number unique in the United States?

ACH or ACH is a methodology on how electronically submitted cheques are cleared. There is no ACH Account Number per se. There is just your regular Bank Account Number. Bank Account numbers are unique only with the bank itself. It could very well be that two banks can share the same bank account number.

The differentiator is not singular. With each Bank Account Number, there is the Routing Number (ABA) and the name associated with the Account (these three items together make it unique - so to speak).