I had 9 rags in two piles, 6+3, in my inventory. Tried to combine them by pulling the pile of 6 on top of the pile of 3 (combine action). The inventory now showed them as two piles of 6. When dragging one of the piles to a new inventory slot, the counter dropped from 6 to 3 again. Looked like duping, but in the end I had the same number of rags.
I can reproduce it consistently with rags but have not encountered it with other items, such as .380 rounds.
Stress test offline version 0.63.146566