漁夫の利を占める。

A third party makes off with the profits.

他の物が漁夫の利を占める。

Someone else profits from the situation.

本命と対抗馬が競り合ったお陰で、ダークホースの彼が漁夫の利を得たってわけだ。

Since the favorite and his rival were running against each other a dark horse like him was able to pull out in front.