Current mangas

 Spoiler

Alright, let's think about this theory, shall we?

If there are twin Yunos, so to speak, with the one running around being twin A and the corpse being twin B, then why does the diary still work? A diary will cease to operate when its owner is dead- and they also address this recently with the eighth dying and all of her proxy diaries losing ability to function. It just wouldn't be possible, unless they were meant as "twin diaries" similar to how the Couple Diary that the 7th had works: though it was a tad fuzzy on how that worked. We were never certain on the idea of if one dies, then would the other partner diary work? Mostly because there's leeway to interpret if Ai was pretty much as good as dead or was dead when she got to Marco.

Regardless, the real nail in the coffin is that if you revisit the scene in Mirai Nikki: Paradox when Akise Aru goes into the memory bank of Mecha-Murumuru, when Murumuru gives Yuno the diary, we see the other Yuno body already dead. There's no way that the body lying on the floor could have possibly been a diary holder given that she didn't even live to the start of the survival game.

You might just be reading too much in it. The author likely put everything Akise was considering before he realized the truth for a reason: he wanted the reader to tie everything that we know together and hit the obvious conclusion.

Also, I was shocked that the author remembered the mascot suit from when Yuno was stalking Yuki prior to the beginning of the series and showed it hanging when Yuno walked into the room with the other Yuno sitting down. From Chapter 34, if you're curious. Yuno's flashback in that chapter was pretty funny and really HHHNNNNGGGGHHH.

 Something Else to Think About

On another note, how do you guys see this series ending? It seems like we might be in for a bad end- with a perpetual Endless Eight situation continuing over and over with them having to play the game over and over again. However, I'm sure that things will turn around and we'll get at least a good ending- I'm sure that all the new things that have been learned from the game prior to the "beginning of the series" and during this game will ultimately lead to an ending where the game doesn't continue and they either reset and somehow don't have to continue the game or they don't reset. I'm very curious how the author could achieve this end though. Perhaps we'll see: "WAIT, YUNO-- I HAVEN'T DONE MY SUMMER HOMEWORK!" pop out of nowhere, and the cycle will end for good. On a more serious note, the "bound together with Yuno" prophecy might lead to what we've been thinking in this thread: Yuno and Yuki merge into a single consciousness and they become God together.

Well..

 Spoiler

As you've stated, when a person dies, their diary is gone, correct? But didn't you noticed how there are really unique diaries that popped up near the end? Ex: Mayor's, etc. I'm unsure on whether if there's such of a thing as a "Twin Diary", and if there are, let's say there are two. What if:

>Yuno A dies, Diary A still exists because it's a special diary that might only cease to exist once both Yunos dies?
>Yuno B grabs Yuno A's diary and uses it as a 'fake'.
>Yuno B now ends up with a total of 2 diaries

Though my theory is unlikely, I still have my suspicions. One of the images in the recent chapter reveals two Yunos, though, noting that it was not an actual event but more so 'what could have happened', the only two likely scenarios that makes sense to me:

a) Alternate-universe/world Yuno
b) Twins with special diaries.

We'll see how this unravels. If I'm wrong, so be it, but I hope I will be blown away by the truth.