1function firstDuplicate(a) {
2    let data = [];
3    
4    for (dup of a) {
5        if (data[dup]) {
6            return dup
7        } else {
8            data[dup] = dup
9        }
10    }
11    return -1
12}

1import java.util.*; 
2  
3class Main 
4{ 
5    // This function prints the first repeating element in arr[] 
6    static void printFirstRepeating(int arr[]) 
7    { 
8        // Initialize index of first repeating element 
9        int min = -1; 
10  
11        // Creates an empty hashset 
12        HashSet<Integer> set = new HashSet<>(); 
13  
14        // Traverse the input array from right to left 
15        for (int i=arr.length-1; i>=0; i--) 
16        { 
17            // If element is already in hash set, update min 
18            if (set.contains(arr[i])) 
19                min = i; 
20  
21            else   // Else add element to hash set 
22                set.add(arr[i]); 
23        } 
24  
25        // Print the result 
26        if (min != -1) 
27          System.out.println("The first repeating element is " + arr[min]); 
28        else
29          System.out.println("There are no repeating elements"); 
30    } 
31  
32    // Driver method to test above method 
33    public static void main (String[] args) throws java.lang.Exception 
34    { 
35        int arr[] = {10, 5, 3, 4, 3, 5, 6}; 
36        printFirstRepeating(arr); 
37    } 
38}