An overlap detection unit for a user station of a serial bus system. The overlap detection unit includes a collision detection block for detecting bus states on a bus of the bus system, in which, in order to transmit a message, bus states of user stations of the bus system are generated on the bus with a first physical layer in a first communication phase, and are generated with a second physical layer in a second communication phase, the second physical layer being different from the first physical layer. The collision detection block generates a signal whose value indicates whether or not the bus states in the second communication phase have a level that corresponds to an overlap of the first and second physical layers or an overlap of two second physical layers, and the collision detection block is designed to output the signal for the user station.