The UID uniqueness plug-in was not designed for use in a multimaster replication scenario. Because multimaster replication uses a loosely consistent replication model, simultaneously adding the same attribute value on both servers will not be detected, even if the plug-in is enabled on both servers.
However, you can use the UID uniqueness plug-in if the attribute on which you are performing the uniqueness check is a naming attribute, and the uniqueness plug-in is enabled for the same attribute in the same subtrees on all masters.
When these conditions are met, uniqueness conflicts are reported as naming conflicts at replication time. Naming conflicts must be resolved manually. For more information, refer to Solving Common Replication Conflicts.