User account has been merged with another "incorrect" account - how can this be prevented? (4351795)

Target user has been merged with an incorrect account from the source domain.
Seems directory synchronization matched the accounts.
What can be done to prevent this from happening with other objects?

Before starting with migration source and target environments should be analyzed, duplicate accounts should be located and appropriate steps should be taken, eg. accounts should be renamed or excluded from migration/synchronization.

QMM Best Practices contains the following information:
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Handling Accounts with Duplicate Names

There may be user accounts already created in the target Active Directory domain with names similar to source accounts. If object matching by name is turned on for the domain pair (the rule is turned on by default), Migration Manager considers two objects to be duplicates if the source object's SAMAccountName is equal to the target's SAMAccountName.

Although Migration Manager reports on duplicate accounts during the migration session and allows you rename, merge, or skip them on the fly, the best practice is to handle duplicates before migration. You should find duplicate user and group names and determine which accounts belong to the same person and which do not. If two accounts, source and target, have the same name and belong to the same person, they should be merged during migration. If not, you should either rename one of them or skip them during migration.

To find duplicate user and group names, use the Duplicate Users and Duplicate Groups reports from Quest Reporter.
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If some of the accounts have been already mismatched please see the following Quest KB article (KB32553) to resolve the problem:

https://support.quest.com/migration-manager-for-ad/kb/32553