SWIFT MT Message SEME Uniqueness

As far as I understand, the SEME (sender's message reference) attribute of a SWIFT MT message has to be unique per sender (it is the sender's responsibility to ensure uniqueness). So, let's say there are two sending entities A and B and I am the receiver. Then it would be a valid scenario to have both of them send out a SWIFT MT message with the same SEME, right? How could I differentiate messages coming from sender A and B? What is common practice in such a case?

In the documentation (MT 558 in that case) it says:

Reference assigned by the triparty agent to unambiguously identify the message.