It could be you will find this question distressing, but please bear with me. I got it from a list of 'Dissertations with Titles That Warrant a Double-Take'. The one that intrigued me most was this:
I Am You, You Are Me: A Philosophical Explanation of the Possibility That We Are All the Same Person
Once you start to think about it, you quickly see that you cannot but take it seriously.
I put it to you first that in a world in which there is a plurality of persons it must be possible that some of these persons are the same as others of them. Why so? Well, because for any putative pair of persons they will turn out to be the same person if there is no significant difference between them. But, you might ask, if there's no significant difference between them how do they appear to be two persons in the first place so that they can then turn out not to be two persons but the same person? How that happens is that the difference between them is revealed to be an insignificant one. Thus, suppose that someone suggests you are the same person as Simon Jenkins or, more alarming still, Mary Chapin Carpenter. Protest as you might, you won't have a leg to stand on if both your legs are shown to be the very same legs, and your writing mind or singing voice is shown to be the same mind or voice as that of, respectively, Simon Jenkins or Mary Chapin Carpenter, and if no meaningful difference between you and him or her can be detected. But, but, but, you will reply: the different names are the give-away; if yours is, say, Marmaduke MacDonald or Trudy Nixon, you cannot then be Simon Jenkins or Mary Chapin Carpenter. You will quickly have to concede here, however, that a single individual can take on two names without thereby becoming two different persons.
The above suffices to establish that in a world with multiple persons, it is possible that some of them are the same persons as other apparent persons, and that there are therefore fewer persons than you would think.
Could this, though, go to the limit? Could all of us - us persons - be the very same person? That is the most worrying aspect of this issue. Imagine the persons you would least like to be. Now how to live with the conclusion that you are in fact them - and I mean not just one of them but all of them? What a grim world we'd be living in. But I'm happy to be able to reassure you on this score. Assuming there are persons at all, we could not all be the same one. We could not all be the same person, because if we were all the same person none of us could recognize him or herself, or indeed anyone else, as being a person. The very notion of personhood would make no sense. We might be (if 'we' is even appropriate in this context) a giant slab of butter, or a land mass, or an ocean, but we could not be individuated as even notionally separate persons who could then turn out to be the same person. To be a person requires the existence of other persons. I require a you and you likewise require a me, or at any rate a someone.
I conclude that though you and I might be the same person for some values of 'you' and 'I', we can't all be the same person, and that is some relief.