Idea must be original, copyrightable and fixed in a certain medium of expression- as a story, audio/ video etc. for example.
The expression ultimately can be copyrighted, not the idea.
For eg., you have an idea of writing a book on " two lost brothers" - you may write a story and copyright the same. But that does not stop anyone else writing with the same theme but with storyline, incidents etc. . different than that developed and copyrighted by you.
DISCLAIMER : General information and advice provided is without any warranties as to suitability for any use, correctness and application to any specific case. Please always take proper legal counsel . However, if it helped anyone even a little a " thanks" via the "Thanks Contributor Button " would be appreciated and would encourage me to keep on making efforts :-) . I am also always open to corrections and further learnings from more experienced Seniors here.