By launching the node replication attack, the adversary can place the replicas of captured sensor nodes back into the sensor networks in order to eavesdrop the transmitted messages or compromise the functionality of the network. Although defending against node replication attacks demands immediate attention, only a few solutions were proposed. Most of the existing distributed protocols adopt the witness finding strategy, which selects a set of sensor nodes somewhere as the witnesses, to detect the replicas. However, the energy consumption of the witness finding strategy is remarkably high and even gets worse in mobile networks. In addition, the location information is necessary for each node if the witness finding strategy is applied. In this paper, a novel protocol, called extremely Efficient Detection (XED), is proposed to resist against node replication attacks in mobile sensor networks. The advantages of XED include (1) only constant communication cost is required for replica detection; (2) the location information of sensor nodes is not required. Performance analyses and comparison with known methods are provided to demonstrate the effectiveness of our protocol.