Hi

I want to extract all the rows in a data frame that have duplicates 
for a given column.
I would expect this question to come up pretty often but I have 
researched the archives and surprisingly couldn't find anything.
The best I can come up with is:

x <- data.frame(a=c(1,2,2,3,3,3), b=10)
xdup1 <- duplicated(x[,1])
xdup2 <- duplicated(x[,1][nrow(x):1])[nrow(x):1]
xAllDups <- x[(xdup1+xdup2)!=0,]

This seems to work, but it's so convoluted that I'm sure there's a 
better method.
Thanks for any help and enlightenment
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