Removing not duplicated rows

As I understand it, you are trying to subset the data frame to include only 
rows with a non-unique id.

Try this:
x <- data.frame(cbind(id=c(1,2,2,2,3,3,4,5,6,6), value=1:10))
id.table <- table(x$id)
x_new <- subset(x, id %in% id.table[id.table > 1])

Jeremy