OK Based on the info if you have separate projects for server and client. I would suggest that you create a project called lets say "Common". Which can have these classes which are common to both projects. This way you will have only one copy.
Both the projects will have to depend on this third project called common.
In case you don't want to or for some reason cannot use a common project you can use separate copies of Person class in different packages. But that is duplication of code and may not be considered best practice. if you can spend time to tweak your project setup so that you have one common project it will pay in long run
I hope it helps :)