fix(chapters): removed duplicate chapters by id (#4810)

fixes #4750

Solved by creating a `Set` for filtering out deplicate elements.

Need confirmation, Shouldn't we add an `assert` for `language` argument
passed to
https://github.com/shaka-project/shaka-player/blob/76f96b9fee2dc43b03f6803dd80c51fdc5b73a9e/lib/player.js#L4340-L4342

AUTHORS

@@ -61,6 +61,7 @@ Prakash <[email protected]>
 Robert Colantuoni <[email protected]>
 Robert Galluccio <[email protected]>
 Rodolphe Breton <[email protected]>
+Rohan Gupta <[email protected]>
 Roi Lipman <[email protected]>
 Roksolana Ivanyshyn <[email protected]>
 Rostislav Hejduk <[email protected]>

CONTRIBUTORS

@@ -91,6 +91,7 @@ Robert Colantuoni <[email protected]>
 Robert Galluccio <[email protected]>
 Rodolphe Breton <[email protected]>
 Rohit Makasana <[email protected]>
+Rohan Gupta <[email protected]>
 Roi Lipman <[email protected]>
 Roksolana Ivanyshyn <[email protected]>
 Rostislav Hejduk <[email protected]>

lib/player.js

@@ -3952,16 +3952,25 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
       return [];
     }
     const chapters = [];
+    const uniqueChapters = new Set();
     for (const chaptersTrack of chaptersTracksWithLanguage) {
       if (chaptersTrack && chaptersTrack.cues) {
         for (const cue of chaptersTrack.cues) {
+          let id = cue.id;
+          if (!id || id == '') {
+            id = cue.startTime + '-' + cue.endTime + '-' + cue.text;
+          }
           /** @type {shaka.extern.Chapter} */
           const chapter = {
+            id: id,
             title: cue.text,
             startTime: cue.startTime,
             endTime: cue.endTime,
           };
-          chapters.push(chapter);
+          if (!uniqueChapters.has(id)) {
+            chapters.push(chapter);
+            uniqueChapters.add(id);
+          }
         }
       }
     }