How to send promo codes generated by an external system

That is a good question! I haven’t actually re-sent coupon codes to those who haven’t used it, but you are correct; the documentation seems to indicate that each person will only receive 1 unique code. When you mentioned sending a second test email, I am guessing you are referring to the Preview & Test option that appears within an email. If so, I ‘m pretty sure that Klaviyo doesn’t consider these “real” emails, so it will continue to send a different code, because it doesn’t keep track of the codes it sent as a test. (not 100% sure about this, but it seems to be my experience.) 

For a “real” test, I would suggest uploading just a few “fake” codes, create a campaign or flow and actually send it to yourself (or whoever needs to see it); then send another campaign (or add a second email to the flow) using the same coupon.

While Klaviyo’s Preview & Test feature is helpful, there are times when I have had to create a campaign or flow and send “real” emails to make sure everything worked correctly. Many times when I create a new flow, I automatically add TEST to the name, and once it works, I clone the flow with the name I really want. :-)