The photo, first posted by Baccarin, shows her set chair emblazoned with the Deadpool name and logo, as well as her character's name, "Vanessa."
Let's do this. pic.twitter.com/pNqcCGReSz— Morena Baccarin (@missmorenab) March 19, 2015
Deadpool's creator, Rob Liefeld, then reposted the photo, adding that Vanessa's first appearance in comics was in New Mutant #98 (also the first appearance of Deadpool himself.
A photo posted by Rob Liefeld (@robliefeld) on
Copycat's real name is Vanessa Carlysle, which alone seems to hint at that being Baccarin's role. Add in the fact that Copycat's first appearance is in New Mutants #98 (though she has shapeshifted into the form of Domino throughout the entire issue, not revealing herself until X-Force #19, a full two years later) and that seems to confirm the role as a sure thing.
Deadpool opens February 12, 2016.