What Negates the Thayammum
All those conditions which call for a fresh bath or a fresh Wudu, render a fresh Thayammum essential for the performance of Salaah and related duties.
So too, when water becomes available to us, Thayammum cannot be resorted to.
If a person finds both his bath and his Wudu negated on any count, it is enough if he/she performs Thayammum just once.