This is probably because the shared label that your collaborator was invited to join, has a name that is same as an existing label in the collaborators mailbox.
In this case, while accepting the invite he would have been asked to change the label name (only) for his mailbox. It is the same shared label with a different name in your collaborators inbox. You can still share and sync emails in the normal way and also, the email ID of the label remains what was originally created by the label admin. It is just that the label will appear with a different name on your collaborators mailbox, so he should preferably name it in a way he can trace its link back to the original.
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