check if Go.tag is this instance of the object

//left click Unit, Select it
if (Input.GetMouseButtonDown(0) && object.tag == “Unit”){
//check it is me???
selected = true;

at the moment all instances get selected.

My best guess is that you should make one or more variables private. The actual selected object should be a static variable which makes a reference to a GameObject.

static var selectedObject : GameObject;

if (selectedObject==this.gameObject)

An even better approach would be to have an overhead which checks if something is selected, then takes over parts of the GameObjects regular script. If you have lots of objects checking "if I am this" you'll notice a slight impact on performance. But instead letting one function control that part for the selected object will make a more clean and effective pipeline.