The skin market is pretty over saturated so there is definitely bound to be some toe stepping and an overlap of similarities in styles and themes. Using similar elements in a new skin is fine, flowers/skull/swords/halos are very popular themes afterall but just don't place them in the exact same place in the exact same way and style.
What Museum did was copy everything ado did and the way ado did it, they didn't just copy the style, and that was wrong of them to do and they should've known better as an artist and art student themselves. The flat colour style doesn't belong to ado either, the only thing that belongs to them is the way they use that style. Only other example I can think of for comparison would be frfrfr that uses that flat, no shade style sometimes but they use it in their own unique way. Ado took the common themes and created new versions of existing ones in their own way, Museum did not.
Re: at what/which point of time does drawing a skin NOT become a copy?
What Museum did was copy everything ado did and the way ado did it, they didn't just copy the style, and that was wrong of them to do and they should've known better as an artist and art student themselves. The flat colour style doesn't belong to ado either, the only thing that belongs to them is the way they use that style. Only other example I can think of for comparison would be frfrfr that uses that flat, no shade style sometimes but they use it in their own unique way. Ado took the common themes and created new versions of existing ones in their own way, Museum did not.
I am hoping this makes sense :')