setnames() on duplicate columns produces unexpected key

[matthewcantele]

library(data.table)

dt <- data.table(a = 1:3,
                 b  = 4:6,
                 b  = 7:9,
                 check.names = FALSE)

setkey(dt, a, b, b)
unique_names <- make.unique(names(dt))
setnames(dt, unique_names)
key(dt) == unique_names
#> [1]  TRUE FALSE FALSE

dt2 <- data.table(a = 1:3,
                  b  = 4:6,
                  b  = 7:9,
                  check.names = FALSE)
setkey(dt2, a, b, b)
nms <- c('blah', 'foo', 'zap')
setnames(dt2, nms)
key(dt2) == nms
#> [1]  TRUE FALSE FALSE

^{Created on 2026-05-08 with reprex v2.1.1}

Session info

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