not-join and mult="first" produces unexpected output

[jangorecki]

library(data.table)
x = data.table(c(1,2,2,2,3,4,4))
y = data.table(c(2,3,4,4,4,5))
x[!y, on=names(x), mult="first"]
#   V1
#1:  1
#2:  2
#3:  2
#4:  4

## expected
#   V1
#1:  1