Count Duplicate Elements.md

function countDuplicate(numbers) {
    // Write your code here
    let counts = {};
    let counter = 0;
    numbers.forEach(function(x) {
        counts[x] = (counts[x] || 0) + 1;
    });
    
    for(let key in counts) {
        if(counts[key] !== 1) {
            counter++;
        }
    }
    
    return counter;
    
}