Scurry County double leasing

We received a lease for our mineral rights last Fall. It was for an area specifically seen in the deed we have in our possession. We have recently received another offer for a lease for land in the same section. When I called the party to explain that it was already leased, he mentioned another part of that section which we have no knowledge of. I asked him to check to make sure but have since received another letter from him involving the whole section and it does not specify what part. Is there a way to find out if there is another part of this section we do not know about and should I be wary about this offer when we seem to already have it leased?