creating alert rules for duplicate entry

Is it possible to create an alert rule that would warn me about creating a sales order with a customer requisition number that was already used on another sales order for the same customer?

For example, I create sales order number SO-000345 for Customer account 2222, and I include 987654 as my customer requisition number
Then I create sales order SO-000390 later in the day for the same 2222 customer, and use 987654 for my customer requistion number again.

I would like to have an alert that says "A sales order for this customer that uses that customer requistion number already exists."

Is this possible to do? I haven't had much luck, and I haven't had any experience working with alerts either. I'm kind of unsure of their capabilities.

Thanks!