rename must not allow duplicated column names

[holgerbrandl]

Expected behaviour: rename should not allow duplicated columns names.

Details

Currently it does:

require(dplyr)
sessionInfo()
iris %>% tbl_df %>% rename(Species= Sepal.Length)

The output is

λ R

R version 3.6.1 (2019-07-05) -- "Action of the Toes"
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Platform: x86_64-w64-mingw32/x64 (64-bit)

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> require(dplyr)
Loading required package: dplyr

Attaching package: 'dplyr'

The following objects are masked from 'package:stats':

    filter, lag

The following objects are masked from 'package:base':

    intersect, setdiff, setequal, union

> sessionInfo()
R version 3.6.1 (2019-07-05)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 16299)

Matrix products: default

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

other attached packages:
[1] dplyr_0.8.3

loaded via a namespace (and not attached):
 [1] tidyselect_0.2.5 compiler_3.6.1   magrittr_1.5     assertthat_0.2.1
 [5] R6_2.4.0         pillar_1.4.2     glue_1.3.1       tibble_2.1.3
 [9] crayon_1.3.4     Rcpp_1.0.2       pkgconfig_2.0.2  rlang_0.4.0
[13] purrr_0.3.2
> iris %>% tbl_df %>% rename(Species= Sepal.Length)
# A tibble: 150 x 5
   Species Sepal.Width Petal.Length Petal.Width Species
     <dbl>       <dbl>        <dbl>       <dbl> <fct>
 1     5.1         3.5          1.4         0.2 setosa
 2     4.9         3            1.4         0.2 setosa
 3     4.7         3.2          1.3         0.2 setosa
 4     4.6         3.1          1.5         0.2 setosa
 5     5           3.6          1.4         0.2 setosa
 6     5.4         3.9          1.7         0.4 setosa
 7     4.6         3.4          1.4         0.3 setosa
 8     5           3.4          1.5         0.2 setosa
 9     4.4         2.9          1.4         0.2 setosa
10     4.9         3.1          1.5         0.1 setosa
# ... with 140 more rows

[hadley]

Simpler reprex:

library(dplyr, warn.conflicts = FALSE)
iris %>% rename(Species = Sepal.Length) %>% names()
#> [1] "Species"      "Sepal.Width"  "Petal.Length" "Petal.Width"  "Species"

^{Created on 2019-12-10 by the reprex package (v0.3.0)}