APL (Dyalog), 43 bytes

'#',{6⍴(≢⍵)⊃'0'⍵,⍵(2/⍵)(∊⍵,¨⊃⌽⍵)(⍵,⌽⍵)⍵}1↓⍞

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APL (Dyalog), 43 bytes

Requires ⎕IO←0 which is default on many systems.

'#',{6⍴(≢⍵)⊃'0'⍵ ⍵(2/⍵)(∊⍵,¨⊃⌽⍵)(⍵,⌽⍵)⍵}1↓⍞

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1↓⍞ drop the first character (the hash)

{ apply the following anonymous function

(≢⍵)⊃ use the length of the argument to pick one of the following seven values:
'0' a zero
⍵ the argument
⍵ the argument
2/⍵ two (2) of each (/) of the argument (⍵)
∊⍵,¨⊃⌽⍵ the flattened (∊) argument (⍵) followed each (,¨) by the first (⊃) of the reversed (⌽) argument (⍵)
⍵,⌽⍵ the argument (⍵) prepended (,) to the reversed (⌽) argument (⍵)
⍵ the argument

6⍴ repeat elements from that until a length of six is achieved

} end of anonymous function

'#', prepend a hash to that