Hello,
Here is my strategy. With local contests (TV/Radio Stations, Newspapers) I only enter once from my computer, my wife enters with another computer and IP Address. The reason being that , the odds are better in winning a local contest, so I do not want to DQ myself. With the national ones listed here, some will not let you enter again ,they tell you, others you just don't know. But I figure with thousands of entries in a national contest, your odds are so low that you could risk it.