": =" changes another data.table

I just stumbled upon some weird behavior in data.table. In short, using ": =" to change (replace) a column value in a data table. The table also changes the values ​​in another data table (which is a copy of the original data table before the operation: =). Sample code below.

Am I missing something fundamental in a great package, or if there is an error report?

Subquery: Iselse () is the best way to change the values ​​as shown below (in a fairly large table, ~ 10 m rows)? It does the job as expected, fast enough (a few seconds), but with verbose = TRUE data.table complains ("The RHS for element 1. was duplicated. Either a NAMED vector or a reworked RHS list.), And I could not decrypt post so far :)

library(data.table)
options(datatable.verbose=TRUE)
DT1 <- data.table(f=as.integer(c(1,2,1,1,1,2,1)))
DT2 <- DT1

tables()

DT1
DT2
identical(DT1, DT2) # OK, they should be identical.

# I am not sure ifelse() is the best way to do this, but it does what I want, even though data.table complains
DT1[, f := as.character(ifelse(f==1,"a","b"))]

tables()
DT1
DT2
identical(DT1, DT2) # Not OK -- why did DT2 change?

, :

R version 2.15.3 (2013-03-01) -- "Security Blanket"
Platform: x86_64-w64-mingw32/x64 (64-bit)
data.table 1.8.8
All 943 tests in test.data.table() completed ok in 27.869sec

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