September 2016 Fiendish Function

The Fiendish Function posed during September 2016 had the following sample guesses and replies:

• GUESS
• REPLY
• 1
• 12
• 2
• 24
• 3
• 36
• 4
• 48
• 5
• 61
• 10
• 1342

The rule was: "Write the number twice, then write it in base nine".

Suppose the guess was 13:

• Write it twice, giving 1313.
• Write the number in base nine, giving 1718.

So the result is 1718.