Knots do in fact dull any cutter faster than the surrounding wood does because they are harder. Knots don't leave severe nicks though.
The secret to good hand planing is to sharpen often - sharpness is the best thing to improve results. When encountering knots it is helpful to skew the plane a bit and approach the knot at an angle but having the blade sharp is important.
I find that I need to sharpen every time I intend to use the plane if I know that I will use it for more than a few passes. When I say 'sharpen' I don't mean grind, coarse stone, medium stone, fine stone - all that rigamarole. I just mean 'honing', i.e., using the fine stone and then strop it on some leather.
At this point, I'll stop because sharpening is another huge topic and I don't want to go off-topic. Suffice it to say that knots are indeed hard to plane but frequent honing helps.
Side note: when using a surface planer (big power tool), if the knives are not new and sharp, they can rip the knots completely out. You can hear them rattling around inside the planer before then find their way out. This is startling when it happens. I should replace my planer knives more often than I do because in my case, it seems that the rollers do not pull the work through as well if the knives are not sharp. This is probably unique to the planer I have.