We faced the same issue but since it was a sparse scenario with our client they agreed to fix it manually if it occurred.
As per my understanding, for the uniqueness check to function correctly that data has to be committed/present in system before the new request is generated. If its part of the same request it may possibly return the same value for multiple users. If you are doing it in Source create profile you can try using the sourceCheck:true option set and check if that helps!