Javascript : Find the length of duplicated properties value from object

var db = [
        {Id: "201" , Player: "Nugent",Position: "Defenders"},
        {Id: "202", Player: "Ryan",Position: "Forwards"},
        {Id: "203" ,Player: "Sam",Position: "Forwards"},
        {Id: "204", Player: "Bill",Position: "Midfielder"},
        {Id: "205" ,Player: "Dave",Position: "Forwards"},
]

What's the best way to find the length of if you have duplicated value of the properties? Notice I have duplicated value as "Forwards" on second , third and last object I want to something like :

 for (var key in db) {
    var value = db[key];
    if ( length of duplicated values == 3) {
       console.log("we have three duplicated values)
    }
 }

Is it possible to do this while the objects are being looped? Please let me know. Thanks.