List double click function

Srikant -- Actually, when I try your code, it works fine with onDoubleClick.

If it doesn't, at your end, you can try 

list.onChange = function () {
  . . .
}

which in fact is more convenient because the function is triggered by a single click. Quicker, and less risk of repetitive-click syndrome.

P.

It might be easier with InDesign’s dialog’s dropdown, something like this:

var sel = app.documents[0].selection[0].contents;
makeDialog();

var numDrop; 
function makeDialog(){
    var d = app.dialogs.add({name:"Choose an Item", canCancel:true});
    with(d.dialogColumns.add()){
        var nList = ["One", "Two", "Three"];
        numDrop = dropdowns.add({stringList:nList, selectedIndex:0, minWidth:80});
    }
    if(d.show() == true){
        numDrop = nList[numDrop.selectedIndex];
        runGrep();
        d.destroy();
	}
}

function runGrep(){
    alert(sel + numDrop)
}

This stopped working a few years ago. Adobe lost interest and it was never fixed. It probably won't be. There are some bugs against this (see https://indesign.uservoice.com), you can add your voice -- but don't get your hopes high.