I\'ve already seen these answers, so please don\'t link em in solutions:
Installing MongoDB on Ubuntu 16.04 https://unix.stackexchange.com/questions/220
I also had the similar issue and it got resolved for me by doing this
Go to this directory /etc/apt/sources.list.d/
and check if there are anyother mongodb-org-*.list files as it would cause conflicts.
then try to install it again, if again issue arises, then run
sudo apt purge mongod*
sudo apt purge mongodb*
sudo apt purge mongodb-org*
to remove any inconsistency and then go to https://docs.mongodb.com/manual/tutorial/install-mongodb-on-ubuntu/ and follow the procedure.
If you're some kind of Ubuntu Derivatives like Linux Mint, Ubuntu GNOME etc, make sure which Ubuntu version is running on your ubuntu derivative version on wiki before pasting the required commands. As in my case I'm using Linux Mint 17 -> Ubuntu 14.04 and I was installing with Ubuntu 16.04 that made the conflict
Hope this would resolve the issue.