Unexpected result of `sum` when the same column is used in J and by (e.g. `dt[,V1,V1]`)

Start by running the reproducible data.table example with d[, sum(V1), V1, verbose = TRUE] and compare it with the expected sum of 10. Trace how grouped evaluation handles the same column in J and by, then verify that the result and column names are correct and add or update a regression test if the repository provides one.

I (maybe naively) think that the result of d[,sum(V1), V1] should be 10. I scanned through ?data.table and didn't see anything immediately obvious about not using the same column in both J and by (although I realize its probably a silly thing to do and I encountered this by accident).

library('data.table')
d = data.table(rep(1, 10))
d[, sum(V1), V1, verbose = T]
#> Argument 'by' after substitute: V1
#> Detected that j uses these columns: <none>
#> Finding groups using forderv ... forder.c received 10 rows and 1 columns
#> 0.000s elapsed (0.000s cpu) 
#> Finding group sizes from the positions (can be avoided to save RAM) ... 0.000s elapsed (0.000s cpu) 
#> lapply optimization is on, j unchanged as 'sum(V1)'
#> Old mean optimization is on, left j unchanged.
#> Making each group and running j (GForce FALSE) ... 
#>   memcpy contiguous groups took 0.000s for 1 groups
#>   eval(j) took 0.000s for 1 calls
#> 0.000s elapsed (0.000s cpu)
#>    V1 V1
#> 1:  1  1
sessionInfo()
#> R version 4.1.1 (2021-08-10)
#> Platform: x86_64-w64-mingw32/x64 (64-bit)
#> Running under: Windows 10 x64 (build 19043)
#> 
#> 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] data.table_1.14.3
#> 
#> loaded via a namespace (and not attached):
#>  [1] ps_1.6.0          digest_0.6.27     withr_2.4.2       magrittr_2.0.1   
#>  [5] reprex_2.0.1      evaluate_0.14     highr_0.9         stringi_1.6.1    
#>  [9] rlang_0.4.11      cli_2.5.0         rstudioapi_0.13   fs_1.5.0         
#> [13] rmarkdown_2.11    tools_4.1.1       stringr_1.4.0     glue_1.4.2       
#> [17] xfun_0.23         yaml_2.2.1        compiler_4.1.1    htmltools_0.5.1.1
#> [21] knitr_1.34

^{Created on 2021-10-25 by the reprex package (v2.0.1)}