There are so many reasons it happens (and I honestly don't think it's intentional or even laziness most of times it does happen). I'm generally not even bothered by it anymore.
You've probably posted duplicate deals yourself, or will do so in the future, and may not even realize it.
MHT962 (rep: 3.2k) posted Apr 23, 2011
The main reason is due to Link variations. Meaning that I can post a deal I got in an email, which will have a different link than the original deal on the website. And also some stores have tags associated with where you are getting the link from which "may" also cause a deal to get duplicated.
OrionStar (rep: 23.7k) posted Apr 23, 2011
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