Rattle, 12 bytes

|I=@P[gbb>]`

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Explanation

|              take string as input
 I             split string into characters and store in consecutive memory slots
  =@           set top of stack to the value of the pointer (i.e. length of string)
    P          set pointer to 0
     [....]`   repeat n times where n is the top of the stack
      g        get character in storage at pointer
       bb      add character to print buffer twice
         >     shift pointer right
               [print buffer output implicitly]