Chemist here

If you’re using a metal kettle, reboiling the same water is fine as there is no changes in the water chemistry.

However there are some precautions for plastic kettles as it uses heat resistant plastic, plastic nonetheless will have some traces of chemicals in the water. That’s after you boil the same water multiples times anyway. So you’ll be fine even if you reboil the same water 2-3 times.

This post has been edited by MichaelJohn: Dec 18 2018, 11:47 PM