I know stackexchange organized a competition recently on automatic duplicate detection. I noticed recently that if I ask a question on the english stackexchange site, which could easily be detected by making an appropriate use of dictionary, such as this question:
I was suspecting there could be duplicates and I went through the suggestions without seeing one, yet it could have been guessed with a good synonym detection algorithm. I'm guessing this could be due to the absence or incompleteness of a synonym dictionary.
Also, I don't understand why the first paragraph on a question that has been closed by moderators as 'Exact Duplicate' has the words 'Possible Duplicate'. I think there should be a conformity there.