XMI 2.1: same xmi:id to different elements

[ngong]

My generator builds on the XMI export.

It assumes that xmi:id uniquely identifies an item.
Now I am faced with the fact that the exported XMI contains three different and independent lifelines from different sequence diagrams exposing the same xmi:id.

Now I am not able to determine the sender object to a message (using the sendEvent attribute of the message): its one of the three.

Is this an error in exporting or do I think something wrong here?

Any suggestion welcome.

Rolf