Outlook email display name issue

Hello,

I am working with two people called Stephen, from two separate organisations. Their email addresses are both stephen @ companyname.com. I have them both set up as contacts, with their correct surnames, and when I send emails out to them they display correctly as 'Stephen A' and 'Stephen B' in the send to list. But when I receive an email from them, or one they are copied into, Stephen A shows up as Stephen B. I have tried clearly the autocomplete list in Outlook, and deleting and remaking Stephen A's contact card, neither of which has worked. Is there anything else I can try to sort it out?