library(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

df <- tibble(a = 1, a = 3:2, .name_repair = "minimal")

df
#> # A tibble: 2 x 2
#>       a     a
#>   <dbl> <int>
#> 1     1     3
#> 2     1     2

df %>%
  select(a)
#> Error: Can't bind data because some arguments have the same name

df %>%
  filter(a == 1)
#> Error in filter_impl(.data, quo): Column `a` must have a unique name

df %>%
  mutate(b = 2)
#> Error in mutate_impl(.data, dots): Column `a` must have a unique name

df %>%
  mutate(c = a)
#> Error in mutate_impl(.data, dots): Column `a` must have a unique name

df %>%
  arrange(a)
#> Error in arrange_impl(.data, dots): Column `a` must have a unique name

df %>%
  summarize(b = sum(a))
#> Error in summarise_impl(.data, dots): Column `a` must have a unique name

df %>%
  group_by(a) %>%
  summarize(b = sum(a))
#> Error in summarise_impl(.data, dots): Column `a` must have a unique name

^{Created on 2018-11-19 by the reprex package (v0.2.1.9000)}