Should you check for a duplicate before inserting into a set

As per the docs:

public boolean add(E e)

Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if this set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.

So the add() method already returns you a true or a false. So you don’t need to do the additional check.