A lot of series rip each other off. The Demon Ororon looks exactly like Sanji on the cover of the first book for example. Luffy is basically Goku in terms of personality but that's because Oda's favourite manga is dragon ball. Naruto and 666 Satan look exactly the same when it comes to artwork. You know why? They two authors are brothers who grew up with the same influences. You can find similarities in every manga, if you've read enough.