I don't have any messaging apps and I got double messages a few times the other day (have only had the phone activated for a few days).
On my previous phone (palm centro) anytime someone sent me a text over 160 characters, it'd appear as an image I'd open to see the text. Now on this phone, it appeared as a download button the other day and when I opened it, it would appear twice and chime twice (not sure, but it may not have chimed before that at all).
As a test I just asked a friend to send me a large text. And although it seems it was delayed, as it came after his second text asking if it was big enough, it did not appear twice and it didn't have a download button to open it either.
Too new of a phone to know what's standard practice, but I was getting double messages without GOsms.