How to remove duplicated (by name) column in data.tables in R?

.SDcols approaches would return a copy of the columns you're selecting. Instead just remove those duplicated columns using :=, by reference.

dt[, which(duplicated(names(dt))) := NULL]
#    x
# 1: 1

How about

dt[, .SD, .SDcols = unique(names(dt))]

This selects the first occurrence of each name (I'm not sure how you want to handle this).

As @DavidArenburg suggests in comments above, you could use check.names=TRUE in data.table() or fread()

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