Im gonna write it quite simple here.

Take 2 items with same skin for example an jackhammer (not the default rust skin) -> open skin menu ->put both in -> take 1 out -> All other skins in the menu still show 2 items -> take other skin with 2 times -> now you have 3 items

If needed I can make a video demonstration of that

Sure,a video will be much better

There you go, for stack limits I use StackSizeController

So quiet

Ah, right. That is indeed because of stacks - you should never ever use stacks for items that are repairable or skinnable. This will not be fixed any time soon because such items are not supported by the plugin and technically the game.

TODO though: support stackables