iMessages received from alternating sources?

When receiving messages from a particular contact, they will alternate between being received from an email address or phone number. This only happens with my iPhone and when he messages other people, they do not have the same problem and will only ever receive messages from a phone number. Upon checking the send and receive section in messages, both his phone number and email address are selected but when he tries to delete the email address, there is an error message to say he cannot delete it. Are there any solutions or explanations as to why this only happens with my phone and not any of his other contacts? Also, would it be to do with my recently iPad that uses the same email as my iphone?