Double Down is an alternate universe fanfic by NeonHomer. It is based on the idea of Daria having a twin sister. However, unlike Canadibrit's The Look Alike Series, which is where the idea originally came from, the sister is a full identical twin of Daria Morgendorffer. The fic is still active as of 4/7/12, and is found exclusively on PPMB.
The setting is, of course, Lawndale. Daria discovers she has a full twin sister, named Matilda Seiler. Matilda (who goes by Tilly) gets in touch with Daria via email, and they arrange a meeting. The story deals with both Daria and Tilly dealing with the repercussions of finding each other, including hidden feelings and arguments with parents.
The fic doesn't really line up to the canon Daria universe, but it loosely falls in the 3rd season, before Speedtrapped, but after Daria!.
- [Double Down - Phase One] (link for continuations in thread)