Since the related thread by jasongeek is marked SOLVED, I'm opening a new thread for anyone else experiencing the same problem.
It appears that I'm also getting duplicate emails for the same two email conditions:
1) Received Payment For Order #xxx (TO ADMIN)
2) Receipt for your order #xxx (TO CUSTOMER)
I'm running Classipress 3.5.4 and WP
4.4.2 with Greenbit 1.2.4 child theme. All plugins on the site were disabled for this test.
All other emails relating to the purchase of a 'single ad pack' were fine (single email).
Mods, can you please look into this ASAP?