Just tested this in-game.

Spoiler: Findings

Cows have a chance of dropping 0-2 leather and 0-3 raw beef.
This means that killing 10 cows will give between 0-20 leather and 0-30 raw beef.

Killing cows individually:
7 leather
24 raw beef

Killing cows after splitting each from the reported bug above:
10 leather
18 raw beef

The behavior I saw in-game didn't really reflect any sort of duplication glitch. The cow would split and assume the name of the stack, but that would be it. It simply had the name, but did not behave like a stack. As for the "actual" stack, it would decrease by 1, meaning that the cow that split was properly accounted for.

What is buggy behavior, however, is that not only does the cow split from the stack and assume the name from the stack it was originally in (in this case, a cow that split from a "10x Cow" stack would be named "10x Cow" and the stack would change to become "9x Cow") but also the name tag isn't consumed, it just stays in your inventory.

Please note, the above tests were done with a Name Tag that hadn't been named yet (It was still called "Name Tag"). When I used a named Name Tag, none of this behavior occurred and the Name Tag worked as intended (consumed, properly named and split from the stack).

TL;DR: Duplication? Most likely not. Weird name tag behavior? 100%!

 

Correct, but I wouldn't discount the possibility. Literally only tested for a few minutes.

If you really want a good test, you need my good pal @314.

 

Added to my to-do list.

Thanks for the overview, poke - this matches my initial assumptions based on a similar¹ bug.

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¹ Actually, it might be the same. Time for some research.