group_by() fails on already grouped data frame when using `.data` pronoun

[kevinykuo]

E.g.

iris %>% 
  group_by(Species) %>%
  group_by(.data$Species)
# Error in mutate_impl(.data, dots) : 
#   Column `Species` can't be modified because it's a grouping variable

[romainfrancois]

Kind of related to #2752

We need some special handling of .data$ in group_by, and/or for mutate to understand that there is nothing to do:

> mutate( group_by(iris, Species), .data$Species )
 Error in mutate_impl(.data, dots) : 
  Column `Species` can't be modified because it's a grouping variable 

[lock]

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