This is an example. R> group <- c(sqrt(2)^2, 2) R> print(aggregate(data.frame(n = seq(group)), list(group = group), length, R+ drop = FALSE), digits = 17) group n 1 2.0000000000000000 2 2 2.0000000000000004 0 Warning message: In `levels<-`(`*tmp*`, value = if (nl == nL) as.character(labels) else paste0(la bels, : duplicated levels in factors are deprecated With sqrt(2)^2 and 2 are considered equal, there is only one group with two members. So, in the result, row 2, with 0 in 'n', should not be there. Compare with the following that uses default 'aggregate.data.frame' (drop=TRUE). R> group <- c(sqrt(2)^2, 2) R> print(aggregate(data.frame(n = seq(group)), list(group = group), length), R+ digits = 17) group n 1 2.0000000000000004 2 R> sessionInfo() R version 3.3.0 (2016-05-03) Platform: i386-w64-mingw32/i386 (32-bit) Running under: Windows XP (build 2600) Service Pack 2 locale: [1] LC_COLLATE=English_United States.1252 [2] LC_CTYPE=English_United States.1252 [3] LC_MONETARY=English_United States.1252 [4] LC_NUMERIC=C [5] LC_TIME=English_United States.1252 attached base packages: [1] stats graphics grDevices utils datasets methods base

(In reply to Suharto Anggono from comment #0) > With sqrt(2)^2 and 2 are considered equal, there is only one group with two > members. So, in the result, row 2, with 0 in 'n', should not be there. Also compare with the following. R> group <- c(sqrt(2)^2, 2) R> print(aggregate(data.frame(n = seq(group)), list(group = as.factor(group)), R+ length, drop = FALSE), digits = 17) group n 1 2 2

From NEWS: "..., duplicated factor levels now produce an error in levels<- instead of a warning, ...." Because of that, the example in Description gives error in R 3.4.0 alpha.