how can i find a mode for only one number?

I am trying to make a function that will find the mode of an array, I have it find the mode but I am still trying to make it that if there are more than 1 modes in the array it will return unavailable.

int amount=0;
int counter=0;
int mode = 0;
for(int i =0; i<size; i++){
    counter=0;
    for(int j=0; j<size; j++){
        if(a[i] == a[j])
            ++counter;
    }
    if(counter > amount){
        mode = a[i];
        amount = counter;
    }
}