Controller area network arbitration with same message id
This question has already been asked here:
Same priority for masters in a CAN network

As previously said, the lower ID is winning the access to the bus.

In the hypothetical cases in which they are emitting at the same time, the frame is going to be corrupted as soon as one ECU write a 1 and get 0 emitted by the other ECU. The error frame is going to be sent!