Spoofing attacks are severe threat to Global navigation satellite system (GNSS). Using the propagation characteristic of spoofing signals, a lot of methods have been put forward to detect and discriminate spoofing. In this paper, we propose a double difference method using Doppler shift measurements of independent receivers, which does not require time synchronization and can be easily integrated on commercial off-the-shelf (COTS) receivers. We derive the theoretical model of Doppler frequency double difference (DFDD) in detail, and analyse the detection performance. Numerical simulations and field experiments are conducted to verify the feasibility of the proposed method.