Add check to prevent a variable to be outcome and predictor at the same time

[EmilHvitfeldt]

There are no checks in add_role() right now, and the following is possible.

library(recipes)

recipe(mpg ~., data = mtcars) %>%
  add_role("mpg", new_role = "predictor")
#> Recipe
#> 
#> Inputs:
#> 
#>       role #variables
#>    outcome          1
#>  predictor         11

This feels like a pattern that shouldn't be possible and would almost always be a mistake.

[github-actions]

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