drake_plan() should look for duplicate targets

[krlmlr]

We still need to double-check for duplicates during make(), keys in tibbles will help (tidyverse/dplyr#1792).

From #304 (comment), CC @rkrug.

drake::drake_plan(a = 1, a = 2)
#> # A tibble: 2 x 2
#>   target command
#>   <chr>  <chr>  
#> 1 a      1      
#> 2 a      2

Created on 2018-03-07 by the reprex package (v0.2.0).

[wlandau-lilly]

Agreed. I'm a bit inundated at the moment, but this should be easy to fix.