What is difference between them?

ona herşeyi anlatmışmış vs ona herşeyi anlatmış vs ona herşeyi anlatmıştı

Also i wonder, is it possible to say anlatmışmıştı?

ona herşeyi anlatmıştı  ---> He/She had  told him/her everything.

ona herşeyi anlatmış  ---> I heard [ I was told that] He/She has told him/her

everything.

 ona herşeyi anlatmışmış --> Colloquial form. "Double mış " reported past is used when you are reporting someone´s action to someone,  in a sarcastic way , its because you are probably in one of the states below;

- you are annoyed/pissed off  with that person

- you don´t belive what he said to you and you think he/she is lying to you

- you don´t care him/her or what he/she done

- you are not very interested in what he/she done

Hastaneye gitmişmiş  de  o yüzden benimle buluşmaya gelmemiş.

He/She said that he/she had gone to the hospital, that´s why he/she couldnt come to meet me.

[which I don´t belive that he/she really gone to hospital so I am annoyed with that ]

So, your sentence may mean ;

ona herşeyi anlatmışmış ---> He/She has told everything to him/her, but I am not really interested in the fact that he/she told him/her everything. [ or , I dont really much care about the fact that he/she told him/her everything]

anlatmışmıştı?  ---> This doesn´t make sense to me.

anlatmıştı and anlattı is the same thing, right?

çok teşekkürler tunci.

No, they are not the same thing.

One is Pluperfect.Turkish style of  Pluperfect is used in Turkish much more than in English.

In Turkish -mIştı  marking is used for actions that are completed in the past. By using - mIştı , we are ;

a) narrating the past action to someone

b) I witnessed or I myself  performed that action.

c)  The action occurred in the definite past *

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whereas, -mış is used to report  the events  which you didn´t  witness. You heard or you were told.

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So,

ona herşeyi anlatmıştı  ---> He/She had  told him/her everything.

 I  witnessed or  I was there while he/she was telling everything to him/her which took place in a certain time in the past.

ona herşeyi anlatmış  ---> I heard [ I was told that] He/She has told him/her

I  heard or I were told that he/she told everything to him/her. But I didn´t witness that.

Hatırlatma (reminding) : Doğru yazım "her şey" (correct orthography is "her şey")