Hello @Peter_Lam
Thank you for contacting the Instructure Community. From what you've explained - that is abnormal behavior as users shouldn't be able to "merge" themselves to other schools or accounts. Was this an admin, teacher, or student? If the SIS ID has been removed or deleted and you need it back, you can run a change_sis_id import that should free it back up. Here is the Canvas guide:
https://community.canvaslms.com/t5/Troubleshooting/Changing-SIS-IDs/ta-p/821
If that isn't the issue, then you may want to contact Canvas Support directly as users in the forum won't really be able to assist without looking at specifics.
Because of the complexity of the issue, and the concerns for privacy, it would be best if you contacted Canvas Support directly to pinpoint the source of the issue and provide a resolution:
How do I contact Canvas Support? - Instructure Community