How to check duplicates in an array with a time complexity of O(n) in C lang?

Question

Well if I use this for example:-

for(i=0 ;i<n ;i++)
{
    for(j=0;j<n;j++)

     {
        if(array[i] != array[j])
         return 0;
     }

}

in this case the time complexity is O(n^2) but how can I check duplicates in an easier and shorter way in order to make the time complexity O(n)? also what does a space complexity of O(k) while k is constant and k is smaller then the inputs size? (k is smaller than n).Thx for any suggestions