Remove control from existing map in onAdd (fixes #3020)

spec/index.html

@@ -30,6 +30,7 @@
 	<script type="text/javascript" src="suites/SpecHelper.js"></script>
 
 	<!-- /control -->
+	<script type="text/javascript" src="suites/control/ControlSpec.js"></script>
 	<script type="text/javascript" src="suites/control/Control.LayersSpec.js"></script>
 	<script type="text/javascript" src="suites/control/Control.ScaleSpec.js"></script>
 	<script type="text/javascript" src="suites/control/Control.AttributionSpec.js"></script>

spec/suites/control/ControlSpec.js

@@ -0,0 +1,29 @@
+describe("Control", function () {
+	var map;
+
+	beforeEach(function () {
+		map = L.map(document.createElement('div'));
+	});
+
+	function onAdd() {
+		return L.DomUtil.create('div', 'leaflet-test-control');
+	}
+
+	describe("#addTo", function () {
+		it("adds the container to the map", function () {
+			var control = new L.Control();
+			control.onAdd = onAdd;
+			control.addTo(map);
+			expect(map.getContainer().querySelector('.leaflet-test-control')).to.equal(control.getContainer());
+		});
+
+		it("removes the control from any existing map", function () {
+			var control = new L.Control();
+			control.onAdd = onAdd;
+			control.addTo(map);
+			control.addTo(map);
+			expect(map.getContainer().querySelectorAll('.leaflet-test-control').length).to.equal(1);
+			expect(map.getContainer().querySelector('.leaflet-test-control')).to.equal(control.getContainer());
+		});
+	});
+});

src/control/Control.js

@@ -37,6 +37,10 @@ L.Control = L.Class.extend({
 	},
 
 	addTo: function (map) {
+		if (this._map) {
+			this.remove();
+		}
+
 		this._map = map;
 
 		var container = this._container = this.onAdd(map),