Original sentence: Finally, Operation Grand Slam was the first first major naval exercise of the newly-formed NATO alliance.Its importance was confirmed when Nazi Germany invaded and occupied Norway during World War Two, establishing naval bases for submarine operations against Allied convoys bound for the Soviet seaport of Murmansk
Edited sentence: Operation Grand Slam was the first first major naval exercise of the newly-formed NATO alliance.This importance was confirmed when Nazi Germany invaded and occupied Norway during World War Two, establishing naval bases for submarine operations against Allied convoys bound for the Soviet seaport of Murmansk