There are a few techs that Frogboy called capstone techs at the end of tech lines that are allowed to be researched over and over again. I'm away from my laptop at the moment or I'd check to see if the one you mentioned is one of those.

No, those are not the capstone techs.  I just checked my in-game tech tree and the capstone techs are things like Manufacturing Stimulus and Research Stimulus, which mention "Capstone" in their tooltips.  Life Support Optimization is a tech that requires you to choose one of three techs, but you can only research one of the three.  Xeno Biology is just a regular tech.  Neither of those should be able to be researched more than once.  If you have a saved game that can reproduce the problem I'd suggest you report it as a bug and supply the saved game through Dropbox or some other file sharing service.

I couldn't reproduce this in a game in 3.1. I suggest you disable mods and verify files after that start a new game and see if this still happens.