We tried reproducing your problem using Arduino 1.8.7 on Ubuntu 18.04 and an A-Star 32U4 but were unable to and programmed the A-Star successfully without any issues. At this point, it seems there might be something wrong with your Linux computer.
Looking at your build output in more detail, the 0x3f portion of the 0x3f0d0d “signature” returned by AVRDUDE is the ASCII question mark, which is sent by the A-Star 32U4 bootloader when it receives an unrecognized command. 0x0d is the ASCII carriage return, which is also sent by the bootloader in response to some commands. Perhaps another program is connecting to the A-Star’s bootloader’s serial port before AVRDUDE and sending some invalid commands to it.
- Amanda