You can build a second Food distribution when you have already upgraded the first to Xeno Irrigation.
This is because in various places in the ImprovementDefs.xml and the ImprovementText.xml files there is a space behind XenoIrrigation; and "XenoIrrigation" does not match "XenoIrrigation ".
The fix is simple: remove the space behind XenoIrrigation in these files.