The similarities between Brian DePalma's misanthropic main character in rock opera Phantom of the Paradise and Darth Vader of Star Wars (and heavy breathing) fame are undeniable, and one of the most overlooked influences in the history of geekdom.
Here's a blurb which describes the Phantom and illustrates the similarities between the two characters:
He’s a character of superhuman power and strength, clad in black leather and a flowing cloak, with a stylized helmet to cover hideous scars on his face. The damage to his vocal cords means he breathes with an eerie rasp, and has a box with lights and switches on his chest to help modulate his voice. He retreats to an egg-shaped chamber where he can meditate and find peace. He’s not wholly evil, but has been enslaved by someone who is: a cruel master who exploits his unique powers in order to further his own, evil ends.
DePalma's Phantom of the Paradise came out in 1974, he conceived of the story well before it was released, and Lucas and DePalma are close friends.
So, is Darth Vader a Phantom ripoff? Lucas ran with a different idea, and created a more fully realized character so I wouldn't say it's a total ripoff. But did the Phantom influence Lucas' "vision", and help bring Darth to the big screen? No question.
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- Godard
Everyone has influences. You just can't do anything that doesn't have elements of something else. Some may be direct influence, or and artist or authors come up with similar ideas independently.
Even in the case of the obvious similarities between C3PO and the lady robot in "Metropolis"... It is not a stretch for any Sci-Fi author to imagine a metal humanoid robot. Just try to come up with something that doesn't look influenced by "Metropolis".
There is still room to be creative, despite similarities to other works. The same people that designed the characters in Star Wars also did the ships and those were certainly quit different from what had been seen before.