[R-Forge #5008] .SD drops columns if name is duplicated

Submitted by: Ricardo Saporta; Assigned to: Arun ; [R-Forge link](https://r-forge.r-project.org/tracker/index.php?func=detail&aid=5008&group_id=240&atid=975)

When duplicate column names exist, `.SD` drops columns

``` S
DT <- data.table(x=1:3, x=11:13)

## Works Fine
DT[, names(DT)=="x", with=FALSE]

## Looses Second Column
DT[, names(DT)=="x", with=FALSE][, .SD]
```
