Creating request using API, duplicate display name

I create new requests using the API. We have some requesters with the same display name, but unique email addresses. From what I can se the only way to specify the requester is using:

        <parameter>
            <name>requester</name>
            <value>Firstname Lastname</value>
        </parameter>

However, when there are two users with the same name all requests get assigned to the first of them.

I want to specify the requester with a mail address since all mail addresses are unique, however when I tested this instead:

        <parameter>
            <name>mail</name> (or email)
            <value> mail@address.com</value>
        </parameter>

the request got assigned to myself instead. And specifying a mail address with the "requester" key word creates a new requester instead of assigning the request to an existing user.

Can anyboy help me getting this to work?

Thanks in advance,

Andreas