[R] extract rows in dataframe with duplicated column values

From: Tiago R Magalhaes <tiago17_at_socrates.Berkeley.EDU>
Date: Fri 18 Mar 2005 - 13:11:59 EST


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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