🐛 Fix video pausing itself after second layout (#32220)

Fixes #32173

1. When closing a container lightbox, the `<amp-youtube>` element is unlaid out. 
2. When opening the lightbox a second time, the `<amp-youtube>` element is laid out again.

The `VideoManager` then incorrectly stores two entries. When playing for a second time it will `pauseOtherVideos()`, incorrectly pausing itself—so the video never plays.

The reused `VideoEntry` maintains its listeners at the e.g. `<amp-video>` level, so they continue to work.

src/service/video-manager-impl.js

@@ -188,6 +188,12 @@ export class VideoManager {
     if (!video.supportsPlatform()) {
       return;
     }
+
+    if (this.getEntryOrNull_(video)) {
+      // already registered
+      return;
+    }
+
     if (!this.viewportObserver_) {
       const viewportCallback = (
         /** @type {!Array<!IntersectionObserverEntry>} */ records
@@ -270,25 +276,35 @@ export class VideoManager {
 
   /**
    * Returns the entry in the video manager corresponding to the video or
-   * element provided
+   * element provided, or null if unavailable.
    * @param {!../video-interface.VideoOrBaseElementDef|!Element} videoOrElement
-   * @return {VideoEntry} entry
+   * @return {?VideoEntry} entry
    */
-  getEntry_(videoOrElement) {
+  getEntryOrNull_(videoOrElement) {
     if (isEntryFor(this.lastFoundEntry_, videoOrElement)) {
       return this.lastFoundEntry_;
     }
 
-    for (let i = 0; i < this.entries_.length; i++) {
+    for (let i = 0; this.entries_ && i < this.entries_.length; i++) {
       const entry = this.entries_[i];
       if (isEntryFor(entry, videoOrElement)) {
         this.lastFoundEntry_ = entry;
         return entry;
       }
     }
 
+    return null;
+  }
+
+  /**
+   * Returns the entry in the video manager corresponding to the video or
+   * element provided
+   * @param {!../video-interface.VideoOrBaseElementDef|!Element} videoOrElement
+   * @return {VideoEntry} entry
+   */
+  getEntry_(videoOrElement) {
     return devAssert(
-      null,
+      this.getEntryOrNull_(videoOrElement),
       '%s not registered to VideoManager',
       videoOrElement.element || videoOrElement
     );

test/integration/test-video-manager.js

@@ -61,6 +61,13 @@ describe
         let video;
         let impl;
 
+        it('should not duplicate entries if laid out twice', async () => {
+          videoManager.register(impl);
+          expect(videoManager.entries_).to.have.length(1);
+          videoManager.register(impl);
+          expect(videoManager.entries_).to.have.length(1);
+        });
+
         it('should receive i-amphtml-video-interface class when registered', () => {
           const expectedClass = 'i-amphtml-video-interface';
           expect(toArray(video.classList)).to.not.contain(expectedClass);