Arbitration and scheduling of multiple tags in state-of-the-art Radio Frequency Identification (RFID) systems is accomplished on the medium access control layer. Currently, only answers of a single tag can be decoded in such a system. If multiple tags respond simultaneously, a collision occurs. In that case, conventional systems discard the physical layer information and a retransmission is executed. This work shows how to recover from such collisions on the physical layer and successfully read the data. The contributions of the paper are: 1) An analysis of the achievable throughput increase of a system, that can recover from collisions at a physical layer is given. 2) A model for a description of collisions on the physical layer is presented. 3) Based on this model, we propose a channel estimation method and two types of receiver structures for separating the signals of a collision of two tags: first, single antenna receivers that discriminate the sources of the two tags in the I/Q plane, and second, multiple antenna receivers which exploit the different spatial signatures of both tags. 4) The functionality of the proposed receiver structures is verified with measurement data of two colliding tags. Eventually, a performance analysis of the receivers is provided.