Hmmm... uses of spring of drowned twin...
R-MULTI 'Neko Senshi' by TheGrum - [Dld Date=01/28/2009] - [Dld Size=455kb] - [Dld Files=20] - [Status=Inactive]
Regrettibly, TheGrum's site is still down... likely will have to try the Internet Archive/Wayback.
R-MULTI 'More Than Half' by Michael Fetter - [Dld Date=10/10/2004] - [Dld Size=702kb] - [Dld Files=5] - [Status=Inactive]
An entry in the Ranma Pigtailed Time Loops Preview thread on TFF uses it. specifically, a entry by Darkfire on 1/29/2010.
R-MULTI 'Pigtailed Time Loops' by Multiple - [Dld Date=11/24/2010] - [Dld Size=2405kb] - [Dld Files=641] - [Status=Not Complete]
Indirectly used in
R-SM 'Of Cats Men and Women' by Innortal - [Dld Date=08/06/2005] - [Dld Size=298kb] - [Dld Files=9] - [Status=Inactive]
There is also Ryoga in:
RANMA 'White Rose' by Aondehafka - [Dld Date=02/13/2004] - [Dld Size=2415kb] - [Dld Files=18] - [Status=Complete]
A sort of Runner up. Ranma uses Ryoko's doppleganger trick in the following to create male/female copies of him/her self:
R-MULTI 'Process of Elimination' by Brian Randall - [Dld Date=08/25/2010] - [Dld Size=756kb] - [Dld Files=9] - [Status=Complete]