I think it enter in one position only if two sites are identical

I think that too

The first electrophile reacts easily in the ortho or para position, but the second electrophile's reaction becomes more difficult.

However, if you have a strong activating group, both an ortho position and a para position will react..  

I think that

Dear Sanad.

Attachment or substitution on the ring totally depends on reaction conditions tuning reaction conditions from low temperature &  low concentration of electrophilic reagent to higher may increase the degree of iodination &  you may  get only ortho,only para ,ortho para and tri-substituted ortho para iodinated ring respectively .%  yield of various products also depends on the conditions too.

With best regards 

Muhammad imran khattak