Find Duplicate in Range 0 and n-1

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    int xorOfRepeatedAndMissing = 0;
    for(int i=1;i<n;i++) xorOfRepeatedAndMissing ^= i;
    for(int i=0;i<n;i++) xorOfRepeatedAndMissing ^= arr[i];
    int difbit = 0;
    int temp = xorOfRepeatedAndMissing;
    while(temp > 0)
    {
        if(temp & 1) break;
        difbit++;
        temp >>= 1;
    }
    int firstNum = 0, secondNum = 0;
    for(int i=1;i<n;i++)
    {
        if(i & (1<<(difbit))) firstNum ^= i;
        else secondNum ^= i;
    }
    for(int i=0;i<n;i++)
    {
        if(arr[i] & (1<<(difbit))) firstNum ^= arr[i];
        else secondNum ^= arr[i];
    }
    for(int i=0;i<n;i++) if(firstNum == arr[i]) return firstNum;
    return secondNum;
}

int main() {
    int n;
    scanf("%d", &n);

    int arr[100];
    for (int i = 0; i < n; i++) scanf("%d", &arr[i]);

    int result = find_duplicate(arr, n);
    printf("%d", result);
    return 0;
}