A Power of Attorney document is valid until it is revoked.
When two such documents are executed, the issue becomes whether or not the second document was intended to revoke and take precedence over the first one.
Theoretically, each document could be valid and the persons appointed in the documents could each have authority to act on behalf of the principal.
If the maker intended to revoke the original Power of Attorney document, a document to do so can be created. See Standard Legal’s Power of Attorney legal forms software page for such a document.