DT.orig <- data.table(A=1:3, B=1:3, C=1:3, D=1:3)
drop1 <- c("A", "C")
drop2 <- c("A", "A", "C")  # note the duplicate value

DT <- copy(DT.orig)
DT[, (drop1) := NULL]
names(DT)
[1] "B" "D"

DT <- copy(DT.orig)
DT[, (drop2) := NULL]
names(DT)
[1] "D"