Humanoid.Changed is firing for Health_XML changes made to the character, as well as Health changes. Essentially firing twice, but once for a property that shouldn’t work with .Changed.
It’s simple enough to repro (shout out to M0rgoth for confirming), just hook up a .Changed even to a humanoid and when the .Health changes it’ll fire once for Health_XML and Health. I’ve attached a place with scripts that will print out the changes, you just have to change Humanoid.Health manually.