Re-boiling water has happened to everyone, maybe unintentionally or unconsciously. However you need to stop it right there as the composition of the water at boiling point is changed and this can be highly dangerous.
At boiling point water turns at boiling point. The steam is make of volatile compounds, burning out of water and turning into gasses, i.e. turn into steam. Once the water cools down and you re-boil it, the dissolved volatile gasses and minerals are subjected to the same process and the chemical components change their structure for the second time. This process is very harmful.