#Java 8, 31 bytes

<!-- language-all: lang-java -->

    s->s.replaceAll("(?s).","$0$0")

[Try it online.](https://tio.run/##jU9NTwIxEL3zK8aGhDZIw1mihsSLB/Hg0fVQS9WyZdrszEKM4bev3bV4UYyXaafvq29jdma2WdedDYYI7ozHjxGAR3bNi7EOVv0K8MCNx1ewslxILfL7YZQHsWFvYQUIl9DR7Ip041LI4mUIUshrUlqci/F8PBeqW/SS1D6HLCnKXfRr2ObkYv74BEZ9xbIjluImZr4DSs7UZ2JIPkIVVrgMFCvcx6amfHh@qxDdPnh01MPix1eHwEH/3aa0fCd2Wx1b1ikDHFCKW0wtX8Ax9hfKREymNM3zNEXct/wfG9RWkvrbq9Q5dJ8)

**Explanation:**

    s->                             // Method with String as both parameter and return-type
      s.replaceAll("(?s).","$0$0")  //  Regex-replace all matches with the replacement
                                    //  And return the changed String as result

*Regex explanation:*

    (?s).                           // Match:
    (?s)                            //  Enable DOTALL mode so newlines are treated as literal
        .                           //  A single character

    $0$0                            // Replacement:
    $0                              //  All characters found in the match
      $0                            //  And again all characters found in the match