Banjo F6 chord (Double C)F sixth on the 5-string banjo: 0 2 2 3, and 6 more shapes

Frets, string 4 to string 1: 0 2 2 3 (open position)

1. String 4 (C): open, C
2. String 3 (G): 2nd fret, index finger (barre), A
3. String 2 (C): 2nd fret, index finger (barre), D
4. String 1 (D): 3rd fret, middle finger, F
5. String 5, the short g: open, G, not in the chord

Sounds: C A D F  ·  F6 is F, A, C, D (1 · 3 · 5 · 6)

The 5th string: the short g adds the 9th over F6, a colour rather than a clash.

Double C tuning: gCGCD, the short 5th string first, then strings 4 to 1.