Most users cannot be bothered to search this considerable pool of ideas b4 posting. Maybe a sort of failsafe could be established like
"Your idea seems to have already been posted (by <user> on <date> and voted by <x users>) would you consider checking whether it differs enough? <Text of similar idea found by server> [acknowledge, discard own and upvote] or [Ignore and continue posting]"
Hopefully this would raise some ideas from the vaults.