This is a much bigger concern for me with academic writing than it is with fanfic. With fanfic, I just trust my instincts - if I write something similar to someone else, I'm not happy about it, but I'm also not going to beat myself up either. With academic writing, it's a lot more pressure, but then I tend to choose strange and obscure topics on purpose, since I'm an early modernist and there's a boat load of work already done in the field on just about everything you can imagine.