SYMPTOM:
When moving users from one project source to another in Strategy Object Manager 9.x, the user receives this message:
The Login (Abbreviation) "<login name>" is used by other user account.
The user move is not completed from that point on.
CAUSE:
This message appears if a user with the same login already exists in the destination project source, but has a different name, as shown in the following image:

In the reverse situation, in which a name is shared but different logins exist, the name that is repeated is changed to "New (1)" in the destination source, shown in the image above with the user 'nametest'.
ACTION:
Ensure that any login that exists in both project sources also shares an identical name.