Does the phrase "duper" is a proper word?
There is a word "duper" though it is not related to this word "super-duper". The word "duper" is used to refer to a person who deceives or tricks someone. It comes from the word "dupe", which means to trick. However it is in no way related to the word "super-duper" which is a compound word, specifically an adjective. It is used to mean that something is extremely large or serious. Ex: This is a super-duper problem. Ex: The song became a super-duper hit.