idempotent
Safe to do twice: doing it again changes nothing.
An operation is idempotent when applying it more than once has the same effect as applying it once -- "set the valve to open" is, "add 5 litres" is not.
Examples
"Fill the tank to 25 L" is idempotent: a second copy finds 25 L already there and does nothing.
"Add 5 L" is not idempotent: if it arrives twice, the plants get 10 litres.