IMO fallout shelter is already a very free to play friendly game and quite a good quality game at the same time:D there is already no option to speed up upgrading processes and 'energy' which limits the amount of time you can play. All of these factors are usually used in Free2play games to help developers profit from their game but fallout shelter does not do that! In not doing that it greatly enhances the joy factor of this game and the freedom to play how you want to. So that brings us down to the question how do the developers make money in this game?
they can't possibly be developing a game for free as nothing is free in this world! You would probably be kinda pissed off too if you had to work for free and cater to the demands of your consumers for free! Bethesda in fallout shelter solves this by introducing lunchboxes which are the only means of getting legendary characters, however there are only a handful of legendary characters to be collected.. So if these legendary characters become unique and can't be duplicated that means the devs will than have even less income to support their game! Which leads to less updates and poorer support to the players! Still keeping in mind that lunchboxes are the ONLY way the devs are making profit and that lunchboxes IMO are already ridiculously cheap to buy considering it's their sole income. If what you guys are demanding comes to fruition than the devs will take a huge blow to their bread and butter:c Think about it guys! Fallout shelter is already an awesome FREE game with no ads no time cap and non of the other annoying money making gimmicks! IMO we should be already greatful for this and not complain so much about this minor issue of duplicated legendaries:) sometimes we gotta think of what the devs want instead of only on what we want! Cheers thanks for reading! TLDR: In my opinion it's not a huge issue!