My own personal favorite answer to the still-unanswered question of 'why the hell are there two 'Team Fortress' both populated with seemingly identical people living seemingly identical lives' is still probably the idea that the Administrator hired 9 mercenaries and then cloned them to fill out both the RED and BLU team.
Like, yeah, I fully admit that it raises a few of it's own unanswered questions (For how long did that cloning technology exist? Did Team Fortress Classic went through the same procedure? What happened to the extra 4.5 mercenaries of each team once Mann vs. Machine and the Comics era happen? Did the Administrator just had them disposed while Gray Mann was on the move for the sake of protecting yet another one of her secrets?) but I also feel like it ties very well thematically with the point of the Administrator's scheme in "The Days Have Worn Away". The most vital part of her revenge is making sure RED and BLU are forever evenly-matched, and that's important enough that she will invest in secret cloning technology and however much shenanigans it takes to ensure neither the mercs nor any of their loved ones realize there's two of each of them running around for the sake of the teams being truly identical.
Also, yeah, it does mean that, like, either the Mercenaries had some sort of Dramatic Clone Reveal that happened totally off-screen at some point during the MvM or Comic-Era and we the readers just totally missed it (Even if all of the Comic-Era Mercs are the originals, realizing that you've spent years of your life fighting your own clone and that they've been also living 50% of your time off with your family is pretty fucked-up) .... or that the Mercs just went on living their lives and never put the pieces together there was some Secret Clone Scheme in the first place...
...But honestly, I think both the idea of Team Fortress never figuring out about the clones, and the idea of Team Fortress figuring it out but getting over it so comically quickly it never comes up again in their misadventures feel so very hilariously in-character for me that I count it as a point in favor of the theory in a way.
Y'know, the idea of the Administrator secretly running both teams was also originally framed as a Mind-Blowing Secret That She Must Keep Secret from the Mercs
And by the end of the comics, the Team either straight up never came to the realization that their old 'enemies' were working for the same employer.... or the realized it at some point during the MvM/Comic Era but it mattered so little to them it never came up on-screen.
And also, I think it's kinda hilarious to think about the concept of like, Merasmus being roommates with the RED Solider
Until he probably got of tired of his shit off-screen and moved to a new apartment............. only to end up with the BLU Solider
And then he moved again.............................. only to find himself with the RED Solider again
I am pretty sure Merasmus is the only person in the TF2 Universe outside of the Administrator and Pauling to know there are two of each Mercenary running around. But he's not bothered by the ethical or philosophical implications of this clone scheme, he is just eternally tormented by the existence of Double Solider.
