Check the D1 on post#39
Then what does the other diode do?
Since the schematic for the relay module is not correct, I suspect there are other errors in your schematic shown in post #1
The other diode is for optional sugarcube relay.
Well as I have already said, according to your schematics, connecting the relay ground to the 8A ground should not make the relay turn on if the code has the pin set to HIGH.
If the schematics are correct, then there must be something wrong with either the relay module, the ATmega8A or one of the power supplies or any combination thereof.
Is it really required to connect both grounds 12v GND and arduino GND together?
No and it is prefered that you don't but the fact the the relay is energized when you do, means something is wrong
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