Idempotent

Idempotent Operations: Operations that have no side-effects if executed multiple times. 
Example: An operation that retrieves values from a data resource and say, prints it 

Non-Idempotent Operations: Operations that would cause some harm if executed multiple times. (As they change some values or states) 
Example: An operation that withdraws from a bank account

http://stackoverflow.com/questions/1077412/what-is-an-idempotent-operation