Have had this problem numerous times.

I am a newbie to this sport of numismatics (just started collecting about a year or so back with the state quarters and some Eisenhower dollars). As an avid surfer of the web, I ran across the Collection Society pages from NGC, and decided to enroll my new collection (just for fun). As I have purchased a majority of my graded coins from dealers and EBay, I have ran across about 5 or 6 times now, where someone else has it in their collection already. The first time, was WOW what a shock. Knock on wood, all times so far, usally within a day or so, I receive an email from Collectors saying that the coin had been released and added to my collection. I reckon to assume that the coin had been part of their collections and are now selling them off, and forgot to clear them in the system.