As of now, you need to give the exact same name to your project as the one that is currently on your AGOL or Enterprise environment in order for it to be overwritten (you are prompted that a project package with that name already exists).
It would be easier if you can choose a project package (from AGOL or Enterprise when sharing with to an Online account, or from you own files when sharing locally) to be overwritten, much like you can with normal services.
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