It can be difficult to credit a design to any particular turner, I frequently do sketches when away from the lathe and the internet often from shapes I observe in nature as I am mostly into organic style work, get home make them let someone see them and they say "Oh yes that is like xyz's work on FaceTube" now I can't honestly say I never saw it, and had an image in my memory, nor can I say that following the flow of the grain would lead many to the same basic curve (lets face it we only have curves and not curves to work with!)
Yes when directly influenced you can say influenced by xyz, but if it is a year or two later and the influence is forgotten but the shape remembered that is less easy.
Personally I shamelessly plagiarise most frequently from seeing a finished shape, I feel the need to make it to see how the cuts get put together to end up there, those will be credited, I also inadvertently plagiarise as I look at a lot of peoples work on the internet, in galleries, shows etc. and some time later I will make something that has a design from my memory but it will be driven by the shape and grain pattern of the wood flowing into the memory of a shape, then I couldn't credit anyone as I wouldn't know where the memory came from.
Multi-part pieces or mechanically functioning pieces may be different.