When I was a student I had a thing about building a library of art books that looked cool (and I would occasionally thumb through for inspiration). To be fair quite a lot of money went into building up my bookshelf.
I would buy art books all the time. Especially if they were discounted. Once I bought the same book twice (without realising). I saw a book that looked interesting, bought it, went home to put it in my bookshelf, which had some sort of ordering, and realised there was another copy of the same book already sitting on the shelf.
The long and short of it? I felt 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 becoming a dumping ground of duplicate information. Its still a dumping ground, but at least one that others might find useful and start following because each pin is unique. It also helps me to find it useful when I check it again down the line.