Over the years, we discovered quite some photos that were already winning entries in one competition but still got prizes from another competitions later that year.... We reported one case to the organiser, they admitted that the competitor really submitted a repeating winning photo. But unfortunately they have no way to avoid this from happening as judges would not be able to read through all possible winning entries in the world. After all, that "wonderful" photo was still a winning photo in that particular competition.
Well, I don't believe there's no way to avoid.... some actions like cancellation of prize, giving warning or black-listing the participants may have some sort of deterrent effect... at least a gesture showing that "we are not fool..."
Recently, we found 3 interestingly wonderful winning entries....
Asia Pacific UW Photo Challenge 2014 Winning Photo - Animal in the Cage
(Regulation: Any images that have won or placed in photo contests with winning entries may not be submitted.)
Our World Underwater 2015 - Honorable Mention - Animal in the Cage
Images that have won or placed in photo contests with winning entries announced before December 1st, 2014, may not be submitted. A winning image is defined as follows: any image that has placed 1st, 2nd, or 3rd in any contest that has resulted in prizes worth $100 USD or more. any image that has placed 1st, 2nd or 3rd in a major photography contest (to be determined subjectively by judges). any image that has placed 1st, 2nd, or 3rd in any photo of the year-type contest. Photos of the day, week, or month are fine, as long as they have not resulted in prizes worth $100 USD or more. any single image that was part of a portfolio, but did not place as a single image, is eligible to be entered.)
Ocean Art Competition 2014 - 5th Place Wide-Angle - Animal in Cage