Popper's find_dupl problem.

hakank.
'''
Next problem:

pos(f([64, 91, 77, 16, 22, 6, 55, 9, 56, 73, 54, 83, 14, 51, 52, 84, 33, 62, 16],16)).
pos(f([43, 43, 88],43)).
pos(f([23, 49, 75, 64, 82, 31, 1, 59, 46, 34, 46, 98, 18, 22, 27, 92, 65, 57, 40, 16, 50, 17, 1, 12, 54, 59, 44, 43, 44, 11, 40, 78, 88, 22, 82, 88, 82, 33, 4, 67, 87, 53, 80, 54, 4, 82, 18],82)).
pos(f([71, 71, 8, 12, 19],71)).
pos(f([89, 64, 27, 42, 50, 98, 4, 76, 37, 12, 9, 36, 52, 97, 11, 93, 3, 53, 100, 78, 101, 36, 92, 87, 59, 31, 68, 76, 83, 4, 34],76)).
pos(f([62, 93, 50, 34, 84, 58, 74, 73, 85, 84, 82, 28, 86, 15],84)).
pos(f([55, 4, 61, 87, 64, 69, 43, 10, 68, 56, 53, 34, 98, 47, 75, 43, 43, 55, 22, 25, 4, 44, 19, 54, 64, 31, 75, 6, 39, 75, 101, 42, 100, 19, 66, 31, 86, 53, 40, 23, 45, 32, 2, 67, 98],75)).
pos(f([79, 4, 5, 82, 28, 7, 7, 7, 98, 23, 49, 41, 64, 75, 79, 5, 66, 74, 7, 33, 101, 94, 18, 86, 10, 78],7)).
pos(f([84, 44, 84, 26, 44, 74, 96, 24, 79],84)).
pos(f([61, 19, 59, 52, 84, 23, 10, 46, 1, 43, 94, 60, 13, 85, 52, 16, 64, 29, 33, 14, 26, 20, 44, 20, 2, 8, 51, 34, 70, 11, 56, 69, 13, 56, 9, 34, 47, 33, 85, 19, 74, 51, 43, 43, 93, 79, 3, 82, 23, 12, 51, 23],56)).
neg(f([76, 45, 73, 41, 29, 52, 78, 27, 99, 79, 94, 47, 48, 61, 4, 33, 98, 77, 70, 47, 83, 88, 36, 57, 3, 96, 73, 97, 1],57)).
neg(f([63, 67, 24, 17, 54, 63, 37, 72, 21, 71, 99, 60, 19, 68, 90, 18, 19, 11, 25, 97, 86, 82],37)).
neg(f([49, 37, 15, 12, 57, 8, 36, 93, 4, 38, 17, 1, 34, 2, 80, 8, 24, 87, 45, 86, 46, 34, 97, 70, 73, 30, 62, 95, 54, 33, 88, 74, 18, 49, 92, 38, 87, 59, 5, 31, 50],4)).
neg(f([18, 18, 32, 34, 27, 15, 97, 15, 50, 23, 79, 22, 93, 48, 77, 40, 87],79)).
neg(f([8, 62, 37, 92, 20, 14, 28, 32, 37, 89, 87, 6, 2, 85, 60, 96, 55, 61, 12, 90, 32, 90, 88, 94, 74, 14, 14, 12, 93, 51, 99, 33, 37, 21, 29, 90, 99, 64, 40, 75],60)).
neg(f([97, 68, 54, 63, 99, 37, 26, 36, 83, 56, 76, 23, 76, 31, 70, 76],31)).
neg(f([81, 76, 91, 64, 100, 8, 68, 80, 54, 35, 83, 39, 85, 95, 26, 4, 12, 78, 45, 8, 59, 20],80)).
neg(f([78, 83, 46, 99, 37, 84, 14, 68, 27, 69, 37, 1, 37, 23, 4, 83, 45, 92, 56, 34, 78, 10, 21, 70, 1],21)).
neg(f([12, 32, 79, 36, 55, 34, 94, 70, 29, 31, 43, 20, 54, 29, 70, 97, 54, 59, 93, 16, 20, 79, 9],9)).
neg(f([57, 53, 27, 28, 65, 65, 46, 74, 9, 70, 58, 98, 64, 86, 88, 2, 1, 21, 56, 95, 95, 4, 66, 65, 90, 22, 66, 100, 96, 49, 15, 86],49)).

Perhaps this is harder. Don't show the detailed analysis or Python code until I ask for it.
'''

GPT4 nailed it:
'''
I have analyzed the provided examples, and I believe the pattern is as follows:

For positive examples, the output is the first number in the input list that is repeated at least once (appears more than once in the list). For negative examples, the provided output does not match the first repeated number in the input list.

Please let me know if you would like to see the Python code for this pattern.
'''

I am impressed that it took it at the first guess.

Python code in find_dupl.py