I want to unite two columns but only when there is a duplicate value in one of the columns. A simple example of the current data:

```
library(tidyverse)
tibble(x1 = c("joe", "ann", "ann", "sam", "sam", "ken"),
x2 = c("boston", "new york", "hartford", "montreal", "boise", "tulsa"))
#> # A tibble: 6 x 2
#> x1 x2
#> <chr> <chr>
#> 1 joe boston
#> 2 ann new york
#> 3 ann hartford
#> 4 sam montreal
#> 5 sam boise
#> 6 ken tulsa
```

And what I want it to look like:

```
#> # A tibble: 6 x 2
#> x1 x2
#> <chr> <chr>
#> 1 joe boston
#> 2 ann - new york new york
#> 3 ann - hartford hartford
#> 4 sam - montreal montreal
#> 5 sam - boise boise
#> 6 ken tulsa
```

I have an mildly onerous workaround but I know there must be a more elegant way.

(This is my first post; sorry for any errors in etiquette )