Put unique elements of a string vector into one long string

paste.unique.string  <- function(x, sep = "/")
{ 
    ## Purpose: Put unique elements of a string vector into one long string. 
    ## Arguments:
    ##    x: a string vector
    ##    sep: what separator to use when pasting all unique elements together. 
    ## Return:  a single string with unique elements of "x". 
    ## Author: Feiming Chen, Date: 19 Jun 2017, 14:00
    ## ________________________________________________
    
    paste0(unique(x), collapse = sep)
}
if (F) {                                # Unit Test
    paste.unique.string(c("a", "b", "a", "b")) # "a/b"
}