Doubled secondary leading tone · Rules

The leading tone of a secondary dominant (for example, the F♯ of V/V in C major) is not doubled: for a moment it is a leading tone.

Doubled secondary leading tone: The leading tone of a secondary dominant (for example, the F♯ of V/V in C major) is not doubled: for a moment it is a leading tone. (Piston, ch. 16, p. 251)