How to retrieve duplicate elements in an array ?

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Javascript

ES6

I was trying to retrieve the duplicate array items using the for a loop. The loop was successful in giving the duplicates but it is giving multiple instances of the duplicates separately ?? How to get single instances of the duplicate array elements in a single new array ??

  let arr = ['A', 'B', 'A', 'C', 'B'];
    console.log(arr.lastIndexOf("A"))
    let result = false;
    // iterate over the array
    for (let i = 0; i < arr.length; i++) {
        // compare the first and last index of an element
        let uniqueChars = [];
        if (arr.indexOf(arr[i]) !== arr.lastIndexOf(arr[i])) {
            uniqueChars.push(arr[i]);

            console.log(uniqueChars)
        }
    }

// Actual output 
['A']'['B']['A']['B']

// Desired output 
['A', 'B']

What I have tried:

Used for loop with the condition to detect duplicate elements. Initiated an empty array with the name uniqueChars.The addition of duplicate elements is taking place with multiple instances. How to get a single instance of duplicate elements in a single array ??