diff --git a/src/observable/map/JoinedMap.js b/src/observable/map/JoinedMap.js
index b50a524a..7d099136 100644
--- a/src/observable/map/JoinedMap.js
+++ b/src/observable/map/JoinedMap.js
@@ -22,27 +22,33 @@ export class JoinedMap extends BaseObservableMap {
         this._sources = sources;
     }
 
-    onAdd(key, value) {
-        this.emitAdd(key, value);
+    onAdd(source, key, value) {
+        if (!this._isKeyAtSourceOccluded(source, key)) {
+            const occludingValue = this._getValueFromOccludedSources(source, key);
+            if (occludingValue !== undefined) {
+                // adding a value that will occlude another one should
+                // first emit a remove
+                this.emitRemove(key, occludingValue);
+            }
+            this.emitAdd(key, value);
+        }
     }
 
-    onRemove(key, value) {
-        this.emitRemove(key, value);
+    onRemove(source, key, value) {
+        if (!this._isKeyAtSourceOccluded(source, key)) {
+            this.emitRemove(key, value);
+            const occludedValue = this._getValueFromOccludedSources(source, key);
+            if (occludedValue !== undefined) {
+                // removing a value that so far occluded another one should
+                // emit an add for the occluded value after the removal
+                this.emitAdd(key, occludedValue);
+            }
+        }
     }
 
-    onUpdate(key, value, params) {
-        this.emitUpdate(key, value, params);
-    }
-
-    onSubscribeFirst() {
-        this._subscriptions = this._sources.map(source => source.subscribe(this));
-        super.onSubscribeFirst();
-    }
-
-    onUnsubscribeLast() {
-        super.onUnsubscribeLast();
-        for (const s of this._subscriptions) {
-            s();
+    onUpdate(source, key, value, params) {
+        if (!this._isKeyAtSourceOccluded(source, key)) {
+            this.emitUpdate(key, value, params);
         }
     }
 
@@ -50,6 +56,49 @@ export class JoinedMap extends BaseObservableMap {
         this.emitReset();
     }
 
+    onSubscribeFirst() {
+        this._subscriptions = this._sources.map(source => new SourceSubscriptionHandler(source, this).subscribe());
+        super.onSubscribeFirst();
+    }
+
+    _isKeyAtSourceOccluded(source, key) {
+        // sources that come first in the sources array can
+        // hide the keys in later sources, to prevent events
+        // being emitted for the same key and different values,
+        // so check the key is not present in earlier sources
+        const index = this._sources.indexOf(source);
+        for (let i = 0; i < index; i += 1) {
+            if (this._sources[i].get(key) !== undefined) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    // get the value that the given source and key occlude, if any
+    _getValueFromOccludedSources(source, key) {
+        // sources that come first in the sources array can
+        // hide the keys in later sources, to prevent events
+        // being emitted for the same key and different values,
+        // so check the key is not present in earlier sources
+        const index = this._sources.indexOf(source);
+        for (let i = index + 1; i < this._sources.length; i += 1) {
+            const source = this._sources[i];
+            const occludedValue = source.get(key);
+            if (occludedValue !== undefined) {
+                return occludedValue;
+            }
+        }
+        return undefined;
+    }
+
+    onUnsubscribeLast() {
+        super.onUnsubscribeLast();
+        for (const s of this._subscriptions) {
+            s.dispose();
+        }
+    }
+
     [Symbol.iterator]() {
         return new JoinedIterator(this._sources);
     }
@@ -74,6 +123,7 @@ class JoinedIterator {
         this._sources = sources;
         this._sourceIndex = -1;
         this._currentIterator = null;
+        this._encounteredKeys = new Set();
     }
 
     next() {
@@ -91,22 +141,140 @@ class JoinedIterator {
                 this._currentIterator = null;
                 continue;
             } else {
-                result = sourceResult;
+                const key = sourceResult.value[0];
+                if (!this._encounteredKeys.has(key)) {
+                    this._encounteredKeys.add(key);
+                    result = sourceResult;
+                }
             }
         }
         return result;
     }
 }
 
+class SourceSubscriptionHandler {
+    constructor(source, joinedMap) {
+        this._source = source;
+        this._joinedMap = joinedMap;
+        this._subscription = null;
+    }
+
+    subscribe() {
+        this._source.subscribe(this);
+        return this;
+    }
+
+    dispose() {
+        this._subscription = this._subscription();
+    }
+
+    onAdd(key, value) {
+        this._joinedMap.onAdd(this._source, key, value);
+    }
+
+    onRemove(key, value) {
+        this._joinedMap.onRemove(this._source, key, value);
+    }
+
+    onUpdate(key, value, params) {
+        this._joinedMap.onUpdate(this._source, key, value, params);
+    }
+
+    onReset() {
+        this._joinedMap.onReset(this._source);
+    }
+}
+
+
+import { ObservableMap } from "./ObservableMap.js";
+
 export function tests() {
+
+    function observeMap(map) {
+        const events = [];
+        map.subscribe({
+            onAdd(key, value) { events.push({type: "add", key, value}); },
+            onRemove(key, value) { events.push({type: "remove", key, value}); },
+            onUpdate(key, value, params) { events.push({type: "update", key, value, params}); }
+        });
+        return events;
+    }
+
     return {
         "joined iterator": assert => {
-            const it = new JoinedIterator([[1, 2], [3, 4]]);
-            assert.equal(it.next().value, 1);
-            assert.equal(it.next().value, 2);
-            assert.equal(it.next().value, 3);
-            assert.equal(it.next().value, 4);
+            const firstKV = ["a", 1];
+            const secondKV = ["b", 2];
+            const thirdKV = ["c", 3];
+            const it = new JoinedIterator([[firstKV, secondKV], [thirdKV]]);
+            assert.equal(it.next().value, firstKV);
+            assert.equal(it.next().value, secondKV);
+            assert.equal(it.next().value, thirdKV);
             assert.equal(it.next().done, true);
+        },
+        "prevent key collision during iteration": assert => {
+            const first = new ObservableMap();
+            const second = new ObservableMap();
+            const join = new JoinedMap([first, second]);
+            second.add("a", 2);
+            second.add("b", 3);
+            first.add("a", 1);
+            const it = join[Symbol.iterator]();
+            assert.deepEqual(it.next().value, ["a", 1]);
+            assert.deepEqual(it.next().value, ["b", 3]);
+            assert.equal(it.next().done, true);
+        },
+        "adding occluded key doesn't emit add": assert => {
+            const first = new ObservableMap();
+            const second = new ObservableMap();
+            const join = new JoinedMap([first, second]);
+            const events = observeMap(join);
+            first.add("a", 1);
+            second.add("a", 2);
+            assert.equal(events.length, 1);
+            assert.equal(events[0].type, "add");
+            assert.equal(events[0].key, "a");
+            assert.equal(events[0].value, 1);
+        },
+        "updating occluded key doesn't emit update": assert => {
+            const first = new ObservableMap();
+            const second = new ObservableMap();
+            const join = new JoinedMap([first, second]);
+            first.add("a", 1);
+            second.add("a", 2);
+            const events = observeMap(join);
+            second.update("a", 3);
+            assert.equal(events.length, 0);
+        },
+        "removal of occluding key emits add after remove": assert => {
+            const first = new ObservableMap();
+            const second = new ObservableMap();
+            const join = new JoinedMap([first, second]);
+            first.add("a", 1);
+            second.add("a", 2);
+            const events = observeMap(join);
+            first.remove("a");
+            assert.equal(events.length, 2);
+            assert.equal(events[0].type, "remove");
+            assert.equal(events[0].key, "a");
+            assert.equal(events[0].value, 1);
+            assert.equal(events[1].type, "add");
+            assert.equal(events[1].key, "a");
+            assert.equal(events[1].value, 2);
+        },
+        "adding occluding key emits remove first": assert => {
+            const first = new ObservableMap();
+            const second = new ObservableMap();
+            const join = new JoinedMap([first, second]);
+            second.add("a", 2);
+            const events = observeMap(join);
+            first.add("a", 1);
+            assert.equal(events.length, 2);
+            assert.equal(events[0].type, "remove");
+            assert.equal(events[0].key, "a");
+            assert.equal(events[0].value, 2);
+            assert.equal(events[1].type, "add");
+            assert.equal(events[1].key, "a");
+            assert.equal(events[1].value, 1);
         }
     };
 }