.::This isn't a bug report::.
I had this problem for months, before I finally figured out what the solution was.
IDK if anybody else has this problem, but when using handcent there was a high likelihood of getting duplicate texts... from anybody.
Various rumored solutions include: deleting mms.apk(don't), stopping handcent from save-checking or whatever, and turning off the defualt messaging's notification and auto retrieving messages (this fixes double notifications, not the messages).
Anyways, the fix for this (if anybody has it) is really, reaaaallly simple, and reaaaallly stupid.
The problem (for me) is that handcent might not be set as the defualt messaging app, or a glitch occurred and it forgot or something. So, what to do is, first check and make sure that there is no default messaging app. (go to settings > applications > manage applications > any messaging app; messaging, handcent, chomp > clear defaults.
Then go into "contacts" and try to text somebody, it'll ask what app you want to use, select the default application checkbox and then choose handcent
(you may not have to do the first step, I didn't.)
And IDK if this'll help anybody, but for me it was amazing. hope it helps other people!