Learn Italian: L'attore stesso ha incontrato il suo sosia ieri. - The actor himself met his look-alike yesterday.

The position of stesso changes its meaning. When placed before the noun (lo stesso attore), it means "the same actor". When placed directly after the noun, as in this sentence, it adds emphasis and translates to "the actor himself" (or "in person").

Could I use medesimo instead of stesso in this sentence?

Yes, you could say l'attore medesimo to mean "the actor himself". Medesimo follows the exact same placement rules as stesso, meaning it also goes after the noun for this emphatic meaning. It just sounds a little more formal or literary.

Why does sosia take the masculine article il and possessive suo if it ends in -a?

Sosia (meaning look-alike or double) is masculine here despite its -a ending, originating from a male character's name in an ancient Roman play. It is also invariable in the plural, so "the look-alikes" would be i sosia.

Since sosia means "look-alike", could I use it for a woman too?