... I'm very confused by your wording but there's a manga called "Vanishing Twin" about a girl trying to become her twin sister
i don't think it's possible because the main character cannot be a main character anymore if she is not the focus of the story, or is treated like a secondary character
however, there are mangs that had talentless or useless character being compared to brilliant or popular person, usually a sibling like
http://www.mangago.zone/read-manga/seigi_no_mikata/
http://www.mangago.zone/read-manga/arisa/
(btw, i recommend the j-drama adaptation of seigi no mikata, since the manga did not get finished scanlating, i think)
Guys I know someone already ask this but I can't seem to find where it is .
Guys do you know a manga where MC is not exactly acting as a MC but rather the second MC , where he/she got outshone by someone now she act like the second person or something
Please because I think it would be interesting (づ￣ ³￣)づ