How to use an if statement to fill two columns related to number of occurencies of interested values

I have a dataset and a column that contains the start of sentences, that might occur:

1. once
2. twice

I have introduced two empty columns in it as follows:

data_copy <- data_copy %>% 
  mutate(unique = NA, duplication = NA)

In order to identify those rows that show 1 single repetition and those that instead have two. This is how I have proceeded

data_sent = data_copy$sentence_start 

to get the complete values of rows I am interested in

duplicated = data_copy$sentence_start[duplicated(data_copy$sentence_start)]#%>% as.data.frame() 

to find values that show repetitions.

I have implemented the following code to assign values at unique and duplication columns

for(i in data_sent){
  if(i %in% duplicated){
    data_copy[,16] <- '0'
    data_copy[,25] <- '2'
  } else {
    data_copy[,16] <- '1'
    data_copy[,25] <- '1'
  }
  
}

The loop works by filling the two columns correctly, but the values seem to be overwritten since they are filled completely only with 0 and 2, the first two values of the statements.

A small extract of the dataset is here:

dput(head(dt, 5))
structure(list(cong_cond = c("congruent", "congruent", "congruent", 
"congruent", "congruent"), sentence_start = c("Wojciech zranil sie mocno i spedzil noc w szpitalu pod", 
"Nie chce isc do pracy, zeby wypelnic papierkowa robote, ale", 
"Zacmienie Slonca nastepuje, gdy slonce i ksiezyc tworza", "Laura lubi tworzyc rekodziela i dlatego zapisala sie na", 
"Altówka ma podobna budowe do skrzypiec, ale ma cieplejszy"), 
    presentation_mode = c("spoken", "spoken", "spoken", "spoken", 
    "spoken"), unique = c("0", "0", "0", "0", "0"), duplication = c("2", 
    "2", "2", "2", "2")), row.names = c(NA, -5L), class = c("tbl_df", 
"tbl", "data.frame"))