Earlier today the ROM for the Mi A1 was released to the public. Some russian users on PDA4U confirmed that the Android One firmware is not compatible with the Mi 5X. Yes the hardware between the two devices is identical but the partitions are different between the two devices due to the A1 using the A/B system that the Pixel devices uses. The A/B systems means that when the device gets an update it will be downloaded and installed on the second system partition and when you restart that second parathion will swap to become System A instead of B to make the updating process seamless. Mi A1 is the first device to get this feature aside from the Pixel as far as I know.
So to sum it up
DON'T FLASH THE MI A1 ANDROID ONE FIRMWARE ON YOUR MI 5X, IT WILL BRICK YOUR DEVICE.