Adam is still in a sleeping bag, so no duvet, but he's had a pillow for a few months (since he was maybe 15 months old). We co-sleep most nights (he starts in his cot and comes in with us if he wants, usually around 5am) and we noticed he seemed to like the pillow (we tried to keep him away from it when he was very small). And then he started pulling a soft toy under his head during his daytime naps (when he is always alone in the cot). So I rolled up a cellular blanket and let him rest his head on it, and sometimes he had one of my old flat pillows. Eventually, last week, we bought a proper toddler pillow. He loves it, and sometimes now sleeps the whole night in the cot.
If he'd not shown a liking to lying on something, I wouldn't have bothered, but since he did I'd prefer he lie on a pillow than a toy.