I backed up my saved game but i also tampered with it. I put a nice weapon in stash for all characters, quit game, then did some file replacement to have this end result: have my character still with weapon in inventory and that weapon in stash for all characters (something like duping but with save folder files micromanagement). Problem is, when i returned, the weapon in my inventory has lost minuscule, but noticeable, values in its stats and properties, especially the elemental damage sums. Doing the trick further (3d, 4th extra weapon, to be collected one by one from stash for all characters), led to the weapon still inside inventory to loose stats and have them rounded down, even further.

Is this normal? Is this some "punishment" from trying to dupe? Can it happen outside of dubious actions during normal gameplay? What gives?

Maybe it is - this is not how you're suppose to get itmes, right? :P

But... one thing. Do you always check them with same character? I'm not sure, but... I think some stats depends on your stats, so different chacater could have different resaults.
Or maybe it was in T2? Darn, I'm not sure now.

Nope, same character. Now i already found better item anyway, so i sold to get rid. But man that scared me! I ain't risking further duping anymore!

I will only keep the saved folder backed up every once in a while, so as not to loose my hardcore char!

Why are you playing hardcore, then?

Because in all aRPGs i do that.

Was actually looking for somebody mentioning this here, because I just started playing a few days ago and been noticing a variation in weapon damage. At first it seemed to only decrease, but now I saw it increase too, seems like the damage value is approximate and when I get back to the game after quitting, it can change a bit. Last night I found a weapon that initially had 60-119, today when I get back to game it had 55-109, quit and get back again, 55-110, quit and get back again, 58-116.
So doesn't seem to have anything to do with messing with it, at least, since I don't, also have no mods or anything.

Items' stats are somewhat randomly generated the first time you pick it up.
And each time you load it get regenerated/calculated base on certain numbers.
Anyway, it would not make much change, I believe.

The weapon I finished the game with had 121-242 +14 when I found it, then ended up 111-222 +13, and various values in between since. The various values being for the base damage, as the bonus was always +13, never back to +14 after quitting after first picking it up. Add all the bonuses and that multiplies a few times.
Ah well.