how to initialize array of unknown size in c

I am doing a homework assignment for an intro to programming class in c.

I need to write a program that looks at an int array of unknown size (we are given a initializer list as the test case to use), and determine all the duplicates in the array.

To make sure that an element that was already found to be a duplicate doesn't get tested, I want to use a parallel array to the original that would hold the numbers of all the elements that were duplicates.

I need this array to be the same size as the original array, which of course we don't really know till the initializer list is given to us.

I tried using sizeof() to achieve this, but visual studio says that is an error due to the variable size (const int size = sizeof(array1);) not being constant. Am I not using sizeof correctly? Or is this logic flawed?

Perhaps there is another way to approach this, but I have yet to come up with one.

Here is the code included below, hope the comments don't make it too hard to read.

// Dean Davis
// Cs 1325
// Dr. Paulk
// Duplicates hw

#include <stdio.h>

int main()
{
    int array1[] = {     0,0,0,0,123,124,125,3000,3000,82,876,986,345,1990,2367,98,2,444,993,635,283,544,    923,18,543,777,234,549,864,39,97,986,986,1,2999,473,776,9,23,397,15,822,1927,1438,1937,1956,7, 29,- 1 };
const int size = sizeof(array1);
int holdelements[size]; 
int a = 0; // counter for the loop to initialize the hold elements array
int b = 0; // counter used to move through array1 and be the element number of the element being tested
int c = 0; // counter used to move through holdelements and check to see if the element b has already been tested or found as duplicates
int d = 0; // counter used to move through array1 and check to see if there are any duplicates 
int e = 0; // counter used to hold place in hold element at the next element where a new element number would go. sorry if that makes no sense
int flag = 0; // used as a boolian to make sure then large while loop ends when we reach a negative one value.
int flag2 = 0; // used as a boolian to stop the second while loop from being infinite. stops the loop when the end of hold elements has been reached
int flag3 = 0; // used to close the third while loop; is a boolian
int numberofduplicates=0;// keeps track of the number of duplicates found

for (a; a < size; a++)
{
    if (a == (size - 1))
        holdelements[a] = -1;
    else
        holdelements[a] = -2;
}

while (!flag)
{
    flag2 = 0;
    flag3 = 0;
    if (array1[b] == -1)
        flag = 1;
    else
    {
        while ((!flag) && (!flag2))
        {
            if (holdelements[c] == -1)
                flag2 = 1;
            else if (array1[b] == holdelements[c])
            {
                b++;
                c = 0;
                if (array1[b] == -1)
                    flag = 1;
            }
        }
        while (!flag3)
        {
            if (array1[d] == -1)
                flag3 = 1;
            else if (array1[b] == array1[d] && b != d)
            {
                printf("Duplicate of %d, index %d, was found at index %d.\n", array1[b], b, d);
                holdelements[e] = d;
                d++;
                e++;
                numberofduplicates++;
            }
        }
    }
    b++;
}
printf("Total Duplicates Found: %d\n", numberofduplicates);
return 0;
}