The light source I think is the same. The reason the preview render looks so dark is because the samples are much lower so in the final render more samples pick up the light better.
Perhaps you can check light bounces in your render Tab and reduce them to 1 or 2 because the default is always 12 which is often excessive.
Also the lighting is poor in your scene so it's very noisy. Perhaps if reducing the light bounces helps to preserve the darkness of your scene when more samples are used, you could try using the denoiser to get rid of some of the fireflies amd speckling.
If it is indeed another light source causing the problem, go to your overlay tab and check your lights. The eyeball next your objects means it's visible in the viewport and camera tells you it'll be visible in the final render. Just make sure the eyeball and the camera icons correlate if you have multiple lights.
Hope that helps :)