How to add sequential values to identified duplicates before first character?

I would like to identify duplicates and then add the sequential number before the first character. In the script below i identified the duplicates

I have a dataset that looks like this

col|
X123
X123
X456
X789
X890
X142
X142
X142


df$col<- ifelse(duplicated(df[,c("col")])|duplicated(df[,c("col")],fromLast = TRUE),
                      make.unique(df$col),df$col)

What my script ends up doing is this

col|
X123
X123.1
X456
X789
X890
X142
X142.1
X142.2

What I would like for it to do is

col|
1X123
2X123
X456
X789
X890
1X142
2X142
3X142

>Solution :

Define a function which prepends sequence numbers and then use it with ave.

add_seq <- function(x) if (length(x) == 1) x else paste0(seq_along(x), x)
transform(DF, col = ave(col, col, FUN = add_seq))

giving:

    col
1 1X123
2 2X123
3  X456
4  X789
5  X890
6 1X142
7 2X142
8 3X142

Note

Lines <- "col
X123
X123
X456
X789
X890
X142
X142
X142"
DF <- read.table(text = Lines, header = TRUE, strip.white = TRUE)