Simular numbers in contact list

I have 2 simular numbers in my phone, ie No 1 - 07741234567 and No 2 - 07991234567 thus being the 3rd and 4th digits being different. However if they are both saved into the contact list under different names, neither of them show up with a name on txts or calls. However if only one of them is in the contacts list, by dialing or receiving txts from bothe numbers show up as that same contact even though they arent. I had this issue with 2 of my numbers but have recreated the same senario with another 2 numbers and it does the same. I thought it would see each digit as unique? Your help would be appreciated.