Find the first duplicate element in the linked list

 Example 1
 6 → 4 → 5 → 9 → 3 → 5 → 4 → 9 → NULL
 First duplicate : 4

 Example 2
 6 → 2 → 3 → 3 → 2 → 5 → 3 → NULL
 First duplicate : 2

 Example 3
 2 → 1 → 8 → 4 → NULL
 First duplicate : None

Here given code implementation process.

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