Unable to migrate users who previously participated in another migration project. (4210652)

When attempting to migrate users who were in turn migrated some time ago from another migration project, the following error is experienced:
Error 0xe3000004. Source object matched by the following high-priority matching rule: objectGUID=A94D2E6B5445B64DA424B0336530DD09 has already been matched with another target object by the following low-priority matching rule: objectClass=user

This behaviour is by design. Please refer to the resolution section for more details.

After the first migration was finished, the QMM service attributes were not removed from the target accounts. Now when these target accounts became source objects in a different migration project, QMM is reading the previously populated service attributes, and considering these accounts as already being matched to some other accounts on the target. By default these attributes are:
adminDisplayName and adminDescription or extensionAttribute15 and extensionAttribute14 if the forest schema was extended for Exchange

Please follow Post-Migration activities section of QMM User Guide, Clean Up the Service Attributes Used for Migration.