My sister called me the other day. She needed backup. My niece was writing a story for school, and she was stuck. My sister threw out some ideas, but my niece said, "I can't use those ideas! They are your ideas!"
I told my sister who told my niece that you can take an idea and make it your own. (I mean, obvious exceptions of course. There is Real Life Plagiarism. I'm not going to write about an eleven year-old boy who goes to wizarding school.) So many books that are out there have similar ideas, but they are their own story.
My son and I are working on two different books right now but with the same idea. It's been a lot of fun. His cousin actually made up the character, which my son turned into drawings. My son threw in an angle on said character which I seized. We've bounced ideas back and forth as I work on my chapter book and he works on his ongoing comic book. It's been a lot of fun. In spite of our sharing of ideas, these books are going to be very different.
On the other hand, I've found myself working on a novel with a brilliant, original idea and then come across another book that sounds similar. Yet, its author and me came up with these ideas independently of each other. It's like the Universe is bouncing ideas off of us.