"dedupe by email" not working when sending mailings

I have been utilizing the "dedupe by email" feature as long as it has existed (years, I think). When multiple contacts are included in my mailing who have the same email address, Civi only sends one message to the address.

Now after upgrading to Civi 4.6, (4.6.2, specifically) this feature is not working for me. I'm able to reproduce it consistently using a small group of test contacts. I've tried with/without click/open tracking. Duplicate emails are always sent.