Use Shift > View > Show Exact Duplicate Items as this is normally a more useful selection. You need to manually select all but one of each group to remove. If you have multiple references to the same files then don't send to the recycle bin. Use my DeDuper script if you're not sure or don't want to do it by hand. See this thread for background and please take note of the warning to backup your library before deduping.
(If you don't see the menu bar press ALT to show it temporarily or CTRL+B to keep it displayed)
removing the duplicates will leave 1 copy on the computer.
Not if you delete all the duplicates as displayed by iTunes.
Test your answer. When possible, make sure your Submission works on your own computer before you post it.
It's not often I feel the need to overtly correct someone, but your answer is misleading and could lead to someone erasing content they would struggle to replace.
Thanks for listening.