When creating a variable with the same name as one of the original columns used in unite, the new column is removed (with remove = TRUE).

[theunis]

For example:

unite(mtcars, vs, vs, am)

removes both the vs and am column, while not creating a new column vs.
If this is intended functionality, perhaps a warning message would be appropriate.

[hadley]

It does create the column, but it then immediately removes it. You're currently on your own in tidyr if you create multiple columns with the same name.