The existence of any existent is either intrinsic to it, or not. The first case describes a necessary essence, and the second a contingent one.
A possible essence is one that accepts both existence and non-existence in of itself. By definition then, existence cannot be intrinsic to a possible essence. For if this were the case, then it would be impossible for this possible essence to not-exist.
Given the above, if a possible essence exists, and since its existence cannot be by virtue of what it is, then its existence must be by virtue of that which is other than itself. And so an existent possible essence, is a contingent one
 Contingent because if existence were not intrinsic to an essence, and if this essence existed, then this means that its existence was acquired from something extrinsic to it. Thus, an existent essence whose existence is not intrinsic to it, depends on something other than it to grant it its existence.
 Which would make the possible essence, not a possible essence, since a possible essence accepts non-existence in of itself. This violates the law of identity, and is therefore absurd.