There's a lot of controversy around the particulars of the diagnosis, but the short answer is yes. Until 1994, it was officially known as Multiple Personality Disorder, and colloquially it was called that up until recently. Whilst, as with any mental health condition, there's no definitive way to prove that it exists, enough people claim to have it and present with similar symptoms that it can be considered very much real. Some individuals with DID choose to showcase their lives, and you can find many Youtube videos that introduce the different personalities in their 'system' (a system is the collective term for all the personalities that live in one body).