Is this timer set correctly for Arduino Mega? It should count exactly one second. And why when I print in serial monitor (seconds_passed++) is it incremented by 2 not by 1?

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noInterrupts();
//set and initialize the TIMER1

TCCR1A = 0; // set entire TCCR1A register to 0
TCCR1B = 0; // set entire TCCR1B register to 0
TCCR1B |= (1 << CS12);
TIMSK1 |= (1 << TOIE1);
TCNT1 = 3036;

attachInterrupt(digitalPinToInterrupt(ignition_pin), ignitionSignal, CHANGE);
interrupts();

ISR(TIMER1_OVF_vect) {
  //TIMER1 overflow interrupt -- occurs every 1sec --
  instantSpeed();
  LPG_Consumption();
  LPG_injector_open_duration = 0;
  unleadedConsumption();
  unleadinj_Open_Duration = 0;
  seconds_passed++;
  vss_pulses = 0;
  TCNT1 = 3036;
}