How to copy a dataframe column within the same dataframe?

General Usage
2

Do you mean this?

julia> df = DataFrame(A=1:4, B=["A", "B", "C", "D"])
4×2 DataFrame
 Row │ A      B
     │ Int64  String
─────┼───────────────
   1 │     1  A
   2 │     2  B
   3 │     3  C
   4 │     4  D

julia> df.b_again = copy(df.B);

julia> df.b_again[2] = "E";

julia> df
4×3 DataFrame
 Row │ A      B       b_again
     │ Int64  String  String
─────┼────────────────────────
   1 │     1  A       A
   2 │     2  B       E
   3 │     3  C       C
   4 │     4  D       D
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