Pitting one item against another only makes sense when the two mentioned items are different.
In this description "sb" is short for somebody, and "sth" is short for something. These abbreviations are not widely used, so this will confuse your audience. It would be better to describe a concrete example.
I could pit salads against steak, such that salads are healthier and steaks are tastier. Pitting one item against another doesn't mean that one item will win. It means that we are comparing and contrasting two options against each other.
To pit salads against salads makes no sense, because both the "first" salads and the "second" salads have the same description (and therefore the same attributes) so any kind of contrast becomes impossible.
The phrase "pit X against Y" is used to highlight the contrast between two people or two items in a ways that represents struggle. It forms the idea of a fight between two people or items, but it does not indicate who might have the advantage. Nor does it indicate who will win, it only indicates that the speaker is asking you to consider the two groups in a struggle against each other.