Conditional Baptism

From the Book of Divine Worship - Holy baptism - Rite One.

If there be reasonable doubt whether any person was baptized with Water In the Name of the Father, and of the Son, and of the Holy Ghost, (which are essential parts of Baptism), such person may be baptized in the form before appointed in this Office, saving that, at the immerson or the pouring of water, the Minister shall use this form of words:

If thou art not already baptized, , I baptize thee in the Name of the Father, and of the Son, and of the Holy Ghost. Amen.