I think it boils down to confidence. Have confidence that you have a great idea, and write it. Haters will hate, and there'll always be someone shouting "Copycat!" Besides, the more popular something is, the more hated it will be - somehow the two always match up. But as Jazzabell said above - if you get yourself millions of readers, Hollywood movie deal and millions of fans - quick frankly, who cares what anyone says? Enjoy your success and roll in the cash let the haters hate, they're not the ones with a million dollar book that millions of people loved.
All the plots have been done, but your characters with their distinct personalities and features haven't been done. Your setting with all it's places, features, and monuments haven't been done. Your story hasn't been written until you've finished it. As Wreybies said, quit worrying and write what makes you happy.
There's even a story for that. Arthur C Clarke's The Nine Billion Names of God. Thank you. Well, I'm coming up on sixty-one years of age, and have never stopped reading and learning. But much of my real learning about writing has been in the last few years. Grammar and vocabulary resources can be very dull reading. I'd recommend readng fiction, especially good fiction. Look up words you don't thoroughly know, and don't automatically assume the writer you are reading knows the word perfectly, especially if it is an uncommon one. The sane goes with sentence structures that seem odd to you. It doesn;t come easily or quickly, but it is attainable with patience - which you will need in any case.
This is very true. Think back to basic prompts in English classes, from elementary through college. The teacher would give everyone the same parameters and requirements, often times with a very specific theme, and no students ever came up with the same story. I think it's fairly common for the skeletal structure of stories to imitate others, especially well-known ones, but it's the ways that you branch together your own points for A to B and beyond that make your own work distinctive. If a writer is still uncomfortable with the amount of similarities, then that writer recognizes exactly which areas could be modified if it's really that important to him/her.
