Thank you for the reply, but I'm not sure what "coupled by stray capacitance" means?
And i alsowant to add:
I'm using the same voltage source for my Arduino and the relays, both at 12V. The + is splitted in the relay. But they share ground. So if the back EMF can lead through the ground i have a reason to use the "flyback diode". I dont know enough abouth back EMF, and i will try to do the reasearch. But i'll ask here aswell, can back EMF lead though ground?