Q: i message split thread

Hi everyone i have an issue that i cant seem to work out if it is my iPhone 6 at fault or my friends 5c

occasionally i receive there message split between two threads on my iPhone i have checked the contact card is correct there is only one contact card for this person what settings should i be checking as this starting to drive me crazy.

Any help 

I have the setting in receive set that i receive messages at my phone number and email address and can only start conversation from my number what do i do to stop this?