When the piezo sensor is activated once (1 RISING signal) the Int_D3_Flag is set twice to TRUE. Based on that I assume the interrupt is triggered twice (I cannot find another explanation)
I have another explanation: After handling all the interrupt based code you're setting the EIFR flag again, which triggers the interrupt routine. If you have the external interrupt enabled (so the interrupt handler is called) the flag is cleared by the hardware when entering the interrupt handler. So you must not set it again otherwise the interrupt handler is triggered.