Why does this ability have its mana cost written twice?
It's written like that because that's what the ability means; "Scavenge [cost]" means "[Cost], Exile this card from your graveyard: Put a number of +1/+1 counters equal to this card's power on target creature. Scavenge only as a sorcery.". The italic text in parentheses is called reminder text, and it's there in this case to tell you what scavenge does.