How to verify duplication before append data in r

I would like to append data to my list if I only append a new data.

data <- c("A","B","C")

My function

x<- function(...){
  
  data <- ifelse(... %in% data, append(data, ""),append(data, as.character(...)))
  return(data)
  
}

If I want to append "D" my desire output is

data
[1] "A" "B" "C" "D"
  

But I got this

 data
 [1] "A"

>Solution :

x <- c("A","B","C")
y <- c("D", "A")

union(x, y)

# [1] "A" "B" "C" "D"