I get duplicate entries in thunderbird, which, upon further inspection, have only slight differences in the url, not even anything in the story or title. For example, the same content and title has five different instances in my slashdot folder. The only difference between them is such:
http://rss.slashdot.org/~r/Slashdot/slashdot/~3/Jg5IYuz181M/Has-NASA-Found-the-Lost-Moon-Tapes
http://rss.slashdot.org/~r/slashdot/slashdot/~3/Jg5IYuz181M/Has-NASA-Found-the-Lost-Moon-Tapes
http://rss.slashdot.org/~r/Slashdot/slashdot/~3/Jg5IYuz181M/Has-NASA-Found-the-Lost-Moon-Tapes
http://rss.slashdot.org/~r/slashdot/slashdot/~3/Jg5IYuz181M/Has-NASA-Found-the-Lost-Moon-Tapes
http://rss.slashdot.org/~r/Slashdot/slashdot/~3/Jg5IYuz181M/Has-NASA-Found-the-Lost-Moon-Tapes
Do you see it? It's s/S in the field right after ~r.
This happens all the time