here in the USA most people do not know they have more than one personality as in diagnostics and disorder label. some do know things like knowing since very early childhood they have had voices or others in their heads talking and such but because DID (your term multiple personalities) begins in very early childhood everything that comes with it is just normal to that person.

here in the USA a person .....usually.... finds out that they have DID through their treatment providers after going through a standard psychiatric evaluation for other things like depression or PTSD...

some people do ....suspect.... or self diagnose with DID (your term multiple personalities) by reading and researching online or someone in their life has self diagnosed their friend or family member..... but that does not mean a person actually has it. here in the USA we have many mental disorders, and physical health problems and normal situations that may seem or have the same symptoms as DID.

it is very dangerous to self diagnose, I do not recommend it to any one whether its a friend, family or stranger...

example I know someone who died of a curable, operable problem simply because they self diagnosed by reading online and asking others in an online support group what they had.

I know many people who have self diagnosed and then their researching interferes with the actual diagnostic evaluations to the point where they ended up either not getting right diagnosis or ending up on medications they did not need which caused life threatening situations...

that is why my standard posting always includes if anyone feels they may have any mental problems to contact a treatment provider who can do actual diagnostic evaluations that rule out or in all the many problems that are similar to what you may feel you have.

that said for my own diagnosis I was a college student in a psych class for becoming a mental health treatment provider. one of the requirements of the class was to do a psych eval for all known mental disorders in the DSM (the diagnostic manual that lists all the american recognized mental disorders and the diagnostic criteria that people have to fit in order to be diagnosed with that mental disorder. american treatment providers use this for diagnosing mental disorders, we are now on the 5th addition standards and diagnostics here in america.)

when all the mental and physical health assessments were done my psychiatrist told both my therapist and I, that I had DID, what it meant and what treatment plans were best for me. its been years since then and now all my alters are integrated/ merged back together with me to form one whole person again.