Clone has several meanings in Daria fandom, each of which will be addressed here.
Pseudo-Clones in Canon
In a number of Daria episodes, new characters briefly appeared who had strong physical and behavioral similarities to existing major characters. The largest number of these appeared in "Camp Fear," where parallel beings resembling the Three Js (Billy, Bobby, Benjy) and the Fashion Club (Tracy, Cindy, and Tatiana) were encountered by Quinn Morgendorffer.
Fanfiction Character Clones
Some fanfic writers have created original characters who have strong parallels to existing major characters. In almost every case, the individuals turned out to be previously unknown siblings of the characters they most resembled. Most famous of these was Lynn Cullen, co-heroine of Canadibrit's The Look-Alike Series, who bore a striking resemblance to Daria Morgendorffer and turned out to be Daria's half-sister.
The clone idea can be take to extremes, such as in TAG's "Snowflakes," in which it turns out that both Daria and Jane have "clones" of themselves scattered across the United States, if not the world. This occurs thanks to illegal fertility techniques, employed on an unknowing Helen Morgendorffer, and Vincent Lane's promiscuity.
Clones of Quinn appear in "More Than Anyone Can Take," in Chris Smith's Kain saga. In "Quinnts," a poorly produced fertility drug causes Helen Morgendorffer to give birth to five identical Quinns, who became famous and rich while Daria becomes the marginalized sibling.