Sorry Mohammad,
This is not the case: in that page I have not entered any title. What you see (Categories) is the default one (when the field “text” is left blank), and it should appear only once. The right one is the red underlined; the wrong one is the dark one (below the first occurence).
If you want to see what happen when I put a title, watch for example at: http://www.giuliobolacchi.it/category/linguaggio-della-scienza/analisi-strutture-linguistiche/
“Categories” (the default one) is correctly replaced by “Argomenti del sito” (the title I entered), but the misbehavior of the theme is still here: title doubled!
Please, check more carefully.
Rosanna