Receiving an MMS. Am i doing something wrong iPhone

Hey guys,
Just wondering if anyone knows why the MMS feature does this (on my phone anyway).
When i receive an MMS from someone who is saved in my contacts, i receive the MMS fine (text and photo), but their details show up as their phone number and not their name.
If i dont recognise the number i have to go through my entire contacts until i find them.
I then have to save the number the MMS was sent from (it includes Country Code) under their contact.
Replying to their MMS goes direct to them, but when they reply back to me with an SMS, they appear twice in my messages box. One for the MMS and one for the SMS. Isnt there a way of just having it all combined?
I hope i havent confused anyone.
Mac OS X (10.5.7)