key issue:  using `setattr` to change level names, keeps unwanted duplicates. 


I am cleaning some data where I have sevearl factor levels, all of which are the same, appearing as two or more distinct levels. 
(This error is due mostly to typos and file encoding issues) 
I have 153K factors, and abot 5% need to be corrected. 

**Example**

In the following example, the vector has three levels, two of which need to be collapsed into one. 

      incorrect <- factor(c("AOB", "QTX", "A_B"))   # this is how the data were entered
      correct   <- factor(c("AOB", "QTX", "AOB"))   # this is how the data *should* be
    
      > incorrect
      [1] AOB QTX A_B
      Levels: A_B AOB QTX   <~~ Note that "A_B" should be "AOB"

      > correct
      [1] AOB QTX AOB
      Levels: AOB QTX


The vector is part of a `data.table`. <BR>
Everything works fine when using the `levels<-` function to change the level names. <BR>
However, if using `setattr`, then unwanted duplicates are preserved. 


    mydt1 <- data.table(id=1:3, incorrect, key="id")
    mydt2 <- data.table(id=1:3, incorrect, key="id")



    # assigning levels, duplicate levels are dropped
    levels(mydt1$incorrect) <- gsub("_", "O", levels(mydt1$incorrect))

    # using setattr, duplicate levels are not dropped
    data.table::setattr(mydt2$incorrect, "levels", gsub("_", "O", levels(mydt2$incorrect)))

                    # RESULTS
    # Assigning Levels       # Using `setattr`
    > mydt1$incorrect        >     mydt2$incorrect
    [1] AOB QTX AOB          [1] AOB QTX AOB
    Levels: AOB QTX          Levels: AOB AOB QTX   <~~~ Notice the duplicate level
   

Any thoughts on why this is and/or any options to change this behavior? (ie `..., droplevels=TRUE` ?)
Thanks


--------------

solution

  > mydt2$incorrect
  # [1] AOB QTX AOB
  # Levels: AOB AOB QTX

  m3.drop <- copy(mydt2)
  m3.fact <- copy(mydt2)

  m3.fact[, incorrect:=factor(incorrect)]
  m3.drop[, incorrect:=droplevels(incorrect)]

  identical(m3.fact, m3.drop)
  # [1] TRUE


Warning message:
In `levels<-`(`*tmp*`, value = if (nl == nL) as.character(labels) else paste0(labels,  :
  duplicated levels will not be allowed in factors anymore