The count of duplicate elements is double what it should be - arrays

Question

I am working on an old exam and the problems states that a given array (int zahlen[]={1,4,5,1,5,7,9,2,3,4}) has values that are the same. The task is to replace the values that are the same with '-1'. After each replacement, a given variable, count, has to be increased by one.
My problem is that the variable count is two-times higher than normal (In this case there are only 3 of the same numbers and the variable shows 6.)
The function is called array_unique. I am would be grateful for a brief explanation of my mistake.
Here is my code:
#include <stdio.h>
#include <stdlib.h>
int main()
{
system("chcp 1252");
int zahlen[]={1,4,5,1,5,7,9,2,3,4};
int len = sizeof(zahlen)/sizeof(int);
int erg = array_unique(zahlen,len);
printf("Es wurden %d doppelte Zahlen gelöscht: \n",erg);
printf("Das Array hat nun folgende Werte: ");
printArrayUnique(zahlen,len);
return 0;
}
void printArrayUnique(int *array, int len){
for(int i=0; i<len; i++){
if(array[i]!=-1){
printf("%d ",array[i]);
}
}
}
int array_unique(int *array, int len){
int count=0;
for(int i=0; i<len;i++){
for(int j=i+1; j<len;j++){
if(array[i]==array[j]){
array[j] = -1;
count++;
}
}
}
return count;
}
I have not figured out any other solution to fix the faulty value of count.

Answer 1

The issue is due to the fact that your are counting duplicates more than once; so, when you have found a duplicate entry, you correctly replace that with -1 but then, later in the loops, you will be (potentially, at least) comparing two or more of those -1 values.
Just add a check that either value is not -1 (along with the test for equality) before incrementing the count variable:
int array_unique(int* array, int len)
{
int count = 0;
for (int i = 0; i < len; i++) {
for (int j = i + 1; j < len; j++) {
if (array[i] == array[j] && array[j] != -1) {
array[j] = -1;
count++;
}
}
}
return count;
}
Note also that, as mentioned in the comments, you really do need declarations of your functions before you use them. Add the following two lines before the main function:
void printArrayUnique(int* array, int len);
int array_unique(int* array, int len);