Side missions question

[Vayics]

I have just recently completed the main story, and wanted to tackle all of the side mission content now. I read online that it was possible to do all of these side missions after completing the main story but I'm struggling to find some of these missions for example "what a mess" where you'd need to head to the Janitor's office and collect the mission off of the board, but I must've already collected it during my original playthrough by mistake. Does this mean that I'm going to have to make a new save just for these side missions?