Update:
As Ananda pointed out, you can use the simpler:
DT[, counts := sequence(.N), by = "V1"]
(where DT is as below)
You can create a "counts" column, initialized to 1, then tally the cumulative sum, by factor.
below is a quick implementation with data.table
# Called the column V1
dataset<-data.frame(V1=c("a","b","c","a","d","b","c","a","d","b","c","a"))
library(data.table)
DT <- data.table(dataset)
DT[, counts := 1L]
DT[, counts := cumsum(counts), by=V1]; DT
# V1 counts
# 1: a 1
# 2: b 1
# 3: c 1
# 4: a 2
# 5: d 1
# 6: b 2
# 7: c 2
# 8: a 3
# 9: d 2
# 10: b 3
# 11: c 3
# 12: a 4
