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# [Uiua](https://uiua.org/), 19 bytes (SBCS)

Defines a function that takes the string and returns a string:

```uiua
⊂@#♭⍥⍉⊙(↯2_3)=3⊸⧻↘1
```

[Uiua Pad this online](https://uiua.org/pad?src=0_19_0-dev_1__eJxdzzEKAjEQBdB-TxF2G-3cJGJSCK5hcw3vYhHYQMDS9CtWYit4m38SSSbN-Kf58ODDeIFwE4jX04D8QnogLYh5g_CWF7U9KsQPnl-E-9ghrVgzliT6YVfTC196z2iqIZo4ybkckZy56Royrf8nnfO-TTrP0VpjrCW0xnI81BCWwp8YpdL79gR15uca8ta7H4DkboQ=)

## Explanation

```text
⊂@#♭⍥⍉⊙(↯2_3)=3⊸⧻↘1   "#ABC"    example input
                    ↘1   "ABC"     drop first character (the "#")
                  ⊸⧻     3 "ABC"   push length, keep argument after result
                =3       1 "ABC"   is length equal to 3?
        ⊙(              "ABC"     dip: while ignoring top of stack
           ↯2_3          ["ABC" "ABC"]  reshape to 2x3 array (repeating)
               )         1 ["ABC" "ABC"]  dip finishes
    ⍥⍉                 ["AA" "BB" "CC"] repeat-transpose:
                            top of stack (here: 1) determines how often
                            to repeat applying the next function 
                            (here: transpose). In other words: `if`
   ♭                     "AABBCC"  flatten array
⊂@#                      "#AABBCC" push '#' and join
```