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data.table(vector, matrix) can return duplicate column names #3193
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Activity
Sowewhat related, I have noticed several cases when colnames are duplicated.
These may be some specific cases, when working with toy datasets and colnames V1, V2, ...
Some examples below for information.library(data.table) ## case 1: setDT duplicates V1 and V2 lst = list(V1=1, V2=1, 1, 1) setDT(lst)[] #check.names=TRUE solves this DT = data.table(V1 = c(1L,2L), V2 = 1:9, V3 = round(rnorm(3),2), V4 = LETTERS[1:3]) ## case 2: computation in j duplicates V1 DT[, sum(V3), by = "V1"] ## case 3 (linked to case 2): rollup duplicates the V1 colname and bugs rollup(DT, j = sum(V3), by = c("V4", "V2")) # ok rollup(DT, j = sum(V3), by = c("V4", "V1")) # error # Error in groupingsets.data.table(x, by = by, sets = sets, .SDcols = .SDcols, : # There exists duplicated column names in the results, # ensure the column passed/evaluated in `j` and those in `by` are not overlapping. # So specifying a different name works: rollup(DT, j = .(SumV3 = sum(V3)), by = c("V4", "V1")) # ok@Atrebas thanks for investigation. Groupings sets functions just wraps around ordinary grouping, thus are affected.
Related to #4124
- case 1:
setDTis designed for speed, so the less the checks the better, any checks on quality of data/metadata should be on user side - case 2: has a dedicated issue duplicate names on simple operation #3644
- case 3: inherits from case 2, as explained before.
Reacted by Atrebas- case 1:
This issue is caused by a fact that data.table has check.names=FALSE by default, while data.frame has check.names=TRUE. I filled #4362
I think this is fixed by #4363, though the breaking change means it's an "eventually" thing.
- added a commit that references this issue
on May 22, 2026
Not ideal to be creating
data.tables with duplicate names from the get-go... those names should beV2_1andV2_2i guess.Fixes with
check.namesbut I'm not sure creating duplicates like this by default is the right behavior; my only hesitation is I'm sure there are other more complicated cases that are still creating duplicates even if we fix this.The fix to this specific issue can be very simple, however, just add 7 characters to this line:
https://github.com/Rdatatable/data.table/blob/master/R/data.table.R#L98