I have this: 

```
var mergeUniqueItems = ["-JsDEcxz_ZSGFLKwd1QM", 
"-JsJ2NXGDYKI6QRsuXVK", 
"-JsJ2RK-kOG2eGcG04xF", 
"-JsJ2RK-kOG2eGcG04xF", 
"-JsJ2YLPiP6751zh8geS"]

```

I have used this: 

```
    var duplicateArray = [];
    for (var i = 0; i < mergeUniqueItems.length; i ++){
        for (var j = 1; j < mergeUniqueItems.length; j ++){
            if (mergeUniqueItems[i] == mergeUniqueItems[j]){
                duplicateArray.push(mergeUniqueItems[i]);
            }
        }
    }

    console.log(duplicateArray);

```

Result has turn to be like this: 

```
["-JsJ2NXGDYKI6QRsuXVK", 
"-JsJ2RK-kOG2eGcG04xF", 
"-JsJ2RK-kOG2eGcG04xF", 
"-JsJ2RK-kOG2eGcG04xF", 
"-JsJ2RK-kOG2eGcG04xF", 
"-JsJ2YLPiP6751zh8geS"]

```

When my expectation is duplicate items in 1 array like below:

```
 ["-JsJ2RK-kOG2eGcG04xF"]

```

If there are more than one duplicate value, array should be like this:

```
["-JsJ2RK-kOG2eGcG04xF", "another_duplicate_1", "another_duplicate_2", ...]

```

I don't really know what's wrong with my code, please kindly help. 

Thanks