library(dplyr)
# toy data
zone <- rep(c("A", "B", "C"), each = 3)
shop <- c(1, 2, 3, 1, 5, 7, 3, 8, 9)
df <- tibble(zone = zone, shop = shop)
df
#> # A tibble: 9 x 2
#> zone shop
#> <chr> <dbl>
#> 1 A 1
#> 2 A 2
#> 3 A 3
#> 4 B 1
#> 5 B 5
#> 6 B 7
#> 7 C 3
#> 8 C 8
#> 9 C 9
How can I identify the shops that belong to multiple zones? In the above example, shop 1 belongs to zone A and zone B; shop 3 belongs to zone A and zone C.