Duplicates in linear time

/*
http://stackoverflow.com/questions/5739024/finding-duplicates-in-on-time-and-o1-space

Input: Given an array of n elements which contains elements from 0 to n-1, with any of these numbers appearing any number of times.

Goal : To find these repeating numbers in O(n) and using only constant memory space.

For example, let n be 7 and array be {1, 2, 3, 1, 3, 0, 6}, the answer should be 1 & 3. I checked similar questions here but the answers used some data structures like HashSet etc.

Any efficient algorithm for the same?
*/


#include <stdio.h>

void dupes(int a[], int n)
{
    int i;

    for (i = 0; i < n; i++)
        while (a[a[i]] != a[i]) {
            int tmp = a[i];
            a[i] = a[tmp];
            a[tmp] = tmp;
        }

    for (i = 0; i < n; i++)
        if (a[i] != i)
            printf("%d ", a[i]);

    printf("\n");
}

int main()
{
    int x[] = {0, 6, 2, 3, 4, 5, 2};

    dupes(x, sizeof x / sizeof x[0]);
    return 0;
}