Create duplicate rows based on conditions in R

Using dplyr

library("data.table")
library("dplyr")

#data
dt <- data.table(ID = c("A", "A", "B", "B"),
                 Amount1 = c(100, 200, 300, 400),
                 Amount2 = c(1500, 1500, 2400, 2400),
                 Dupl = c(1, 0, 1, 0))
#result
rbind(dt,
      dt %>% 
        filter(Dupl == 1) %>% 
        mutate(Dupl = 2,
               Amount1 = Amount2))

#    ID Amount1 Amount2 Dupl
# 1:  A     100    1500    1
# 2:  A     200    1500    0
# 3:  B     300    2400    1
# 4:  B     400    2400    0
# 5:  A    1500    1500    2
# 6:  B    2400    2400    2

You could try

rbind(dt,dt[Dupl==1][,c('Amount1', 'Dupl') := list(Amount2, 2)])