As a student, I had a thing about building a library of art books that looked cool (and I would occasionally thumb through them for inspiration). So quite a lot of money went into building up my bookshelf.
I would always buy art books, especially if they were discounted. Once, I bought the same book twice (without realising it). I saw an interesting book, bought it, went home to put it on my bookshelf, and discovered another copy was already on the shelf.
Hmmm…
The long and short of it? I felt like a fool. But at least I could return the new copy and get my money back, and I didn’t have to have a bookshelf of duplicate books.
I noticed that Pinterest has something similar; 
When I tried to add the same pin, it informed me that I had already saved it. This prevents my boards from becoming a dumping ground for duplicate information. It’s still a dumping ground, but at least one that others might find helpful and start following because each pin is unique. It also helps me to find useful pins when I recheck them down the line.