This is why, between two successive measurements, I place one foot on the scale to measure the weight of just one leg (comes to 18-20 lbs). And the following (third) measurement is sufficiently different from the first one. I use this "weird trick" when I'm not happy with the first reading, and want a second opinion ;-)
Oh wow I always wondered why this happened as it felt vaguely weird, I was skeptical that these things were accurate enough to be so repeatable, but never suspected this.
I bought a cheap $20 off amazon and it was seemingly accurate to 0.1 lb. I tested by weighing myself plus a bowl with varying amounts of water and there's no "report the same weight if the measurements are close" behavior you describe.
This happens on mine if I don't let the scale calibrate before the first measure. The scale turns on if you step on it, so it's tempting to just put both feet on in quick succession while the scale it's still off. That will give you a wrong reading. If you step off, the scale calibrates, and the next reading will be right.
You should get two equal readings if for the first one you push on the scale with one feet, take it off, wait a few seconds, then weigh yourself.
Passive damping may be smart, but stupid always wins.