I have an strange theory: Chrom was originally planned as the Fire Emblem character of this game, but Sakurai and his team seeing the people's opinions about him since last year/months made them change their mind about his inclusion, and instead they changed Chrom for Robin. Remember the main argument of the community against Chrom is 'oh not just another blue-haired SWORDSMAN from Fire Emblem', also the whole internet considering Chrom's inclusion a fact thanks to the Gematsu leak caused a lot of hate against him, and if you remember there was stated at E3 that there are people from Nintendo among us (Smashboards and other videogame sites), so maybe knowing this the development team changed their mind and decided to include Lucina as a 'last minute replacement', making her a clone cause they didn't have the time to develop a whole new character having wasted Chrom.
But maybe they finished Lucina before planned and had still some time, so they included Robin, if you payed attention you may remember him stating during his trailer 'I am no ordinary swordsman', making a reference of how people didn't want another swordmand from FE. And Chrom defeated on the floor 'having his chance another day'. But his model was already complete, so they decided to use him as a part of Robin's Final Smash.
If this supposed change of plans ocurred it could have been during the last few months, as they even had to make a CGI trailer for them.
Yeah, this may sound far-fetched for some of you, but I think it could be a possibility.