9.42% - That's the percentage of questions on EL&U.SE that are marked as duplicates. These are other SE sites' duplicate percentages:
Stack Overflow: 2.4%
Super User: 3.26%
Server Fault: 1.66%
A reason for EL&U.SE having so many duplicates is that most questions don't have searchable titles.
An example scenario of why non-search-friendly titles are bad:
I always try my best to find out if my question has already been asked before asking it. I usually can't find an existing one, so I ask my own question. Later on, a notice appears on my question:
Is or are - "The most preferable length of the internship is 3 months but 2 is also acceptable"
Closed as duplicate of
"If your car has 4 wheels, it's drivable. 3 wheels (is/are) a safety hazard." Which conjugation of "to be" is right?
How would I have possibly known about the question that mine was closed as a duplicate of?
Question titles on EL&U.SE either ask about the correct usage of word(s) in a specific context, like this:
"1 or 2 friends {was/were} eating {a sandwich/sandwiches} at the park. Which is correct?"
"The rock was uplifted vs. the rock had been uplifted?"
"What's the prepositional phrase in this sentence: The lights were bright in the room on Monday?"
Or their titles are stupidly vague, like this:
"What is this part of speech called?"
"Why is this word used here?"
"Is this sentence grammatically correct?
When someone does research prior to asking a question, it's highly unlikely that they'll find what they're looking for. And they'll probably write their own question's title with the same issues, later being closed as a duplicate of another question with some obscure title.
Idea:
Edit question titles from this...
Why is it "the tiger is endangered" and not "a tiger is endangered" when referring to all tigers?
..to this...
"When should I use a definite article with a generic noun?"
Should we title our questions to optimize for searchability?