I've recently done a test that wraps and forwards RTP packets (vpn style) as well as sent direct, and I found that the duplicate detection wasn't working. Care was taken to be sure that identical packets with the same timestamp showed up on the same session on the receiving end. The audio sounded, well, like the stack was processing duplicate packets. Audio sounded a bit like a CD skipping.
Anyhow, for what it is worth, oRTP used together with Mediastreamer2 didn't seem to drop duplicates like I expected.
--Matt