eventListener firing twice on mac

                    function mainListBoxEventListener(event){
                        var keyCode = event.keyName;
                        // Check if the key pressed is the Enter key (Key code: 13)
                        if (keyCode === "Enter") {
                            alert("entertest")
                        }else if(keyCode === "Down"){
                            alert("downtest")
                        }else if(keyCode === "Up"){
                            alert("uptest")
                        }else if (keyCode === "Escape") {
                            myPanel.close();
                            return false;
                        }
                        event.stopPropagation();
                    }

                    myPanel.grp.inputGroup.textFieldGroup.httpGroup.httpEditText.addEventListener("keydown", mainListBoxEventListener);

This is a snippet of my code (please ignore the commented section). It runs fine on Windows, with all the keys firing once. However, when I run the same code on mac, it always ends up firing twice, I can't seem to find the root source of the problem other than thinking it's a bug.

Hoping someone can help me here! TIA!