"Uniqueness constraint violation" error while applying O365 license during migration (4277046)

The following error may be observed in the MAgE agent log when migrating to Office 365:

Quest.MigrationManager.Exchange.O365UserProvisioning.O365UserLicenseException: Failed to set license for user - 'alias@domain.com': 'Uniqueness constraint violation.'.

The object in Office 365 remains non mail-enabled.

One of the possible reasons for this is having a mail contact and a user object in the target organization. When MAgE tries to apply a license for the user object, and mail-enable it, Office 365 returns the error due to a conflict between the mail contact and user object.