Unique ID: Not possible to assign an ID for each email address

how can I assign an ID number to each email recipient that shows up on the form response?  Using Sendblaster

Unfortunately there is no way to assign a unique ID for each email address. You could assign an ID to each submission. Because submissions will be unique each time. 

But the email address might not be unique if it was submitted previously. Which would lead to incorrect IDs'. There would be 2 ID's (or more) for each email.

Why don't you just use the email address as the ID? That would work perfectly.