Ki Ga Au To Iu Koto Wa

From Shinmakoku Scanlations :
1-3) Ki ga Au to Iu Koto wa (Hitting it off with you)
Shiraishi has been seduced by his so called “Best Friend” for 6 years since they were both in high school. The train they take to work, their favorite music, their sandwich choice for lunch… always happen to be identical. That strange relationship would never ever change, supposedly. But now that Shiraishi is divorced, it could be just a matter of time before he gets himself scored.
4) 1 + 1 + 1
Between a man who bears a past and a revived lover, the existence of a certain “man” becomes the trigger. Both prickly pain and sweetness are all in one long story…