Let me just reiterate: I am *not* sending from my Gmail account to myself. I am sending test messages from an external account, so suppressed duplicates are not the issue here.
It seems like I'm not the only one experiencing this issue, judging by some of the posts in this thread on WHT:
http://www.webhostingtalk.com/showthread.php?t=1247647
I just wish I could find some kind of documentation that will tell me exactly when Gmail will silently gobble up a message but not deliver it, but I guess that would aide spammers, too, so probably no luck with that.