It is no longer necessary for the value in the Employee ID field of a Person record to be unique. This extra flexibility is provided for large enterprises that have multiple legal entities in different countries.
In such environments, it is likely that the different legal entities use different HR systems, which introduces the possibility that there are multiple employees within the enterprise who have the same employee number. By allowing non-unique employee IDs, it becomes possible for these multinationals to register all their employees in a single 4me directory account and still use the Employee ID field. Even when all these employees are linked to the same Organization record in the directory account, it is still possible to use the Employee ID field without running into issues when the same employee number is used more than once.