Nested For Loops - Check If There Is A Duplicate Element In The Array

Nested for-loops always throw me in a loop.I found a snippet that uses 2 for-loops to check if there is a duplicate element in the array:

/*
* brute force way of checking if array contains duplicates in Java comparing each elements to all other elements of array complexity on order of O(n^2) not advised in production
*/
public static boolean bruteforce(String[] input) {
for (int i = 0; i < input.length; i++) {
for (int j = 0; j < input.length; j++) {
if (input[i].equals(input[j]) && i != j) {
return true;
}
}
}
return false;
}

Let us say we have:
String[] input = new String[] {"one","two","three","one"}