Fix: Discard only invalid overlapping periods

Discard only invalid and/or overlapping periods to fulfil the requirements

lib/dash/dash_parser.js

@@ -101,6 +101,12 @@ shaka.dash.DashParser = class {
      */
     this.largestPeriodStartTime_ = null;
 
+    /**
+     * Period IDs that have seen before.
+     * @private {!Array.<number>}
+     */
+    this.seenPeriodIds_ = [];
+
     /**
      * The minimum of the availabilityTimeOffset values among the adaptation
      * sets.
@@ -581,11 +587,15 @@ shaka.dash.DashParser = class {
       // the last period in manifest
       if (this.largestPeriodStartTime_ !== null && start !== null &&
           start < this.largestPeriodStartTime_ &&
+          !this.seenPeriodIds_.includes(start) &&
           i + 1 != periodNodes.length) {
         shaka.log.debug(
-            'Skipping Period', i + 1, ' as its start time is smaller than ' +
-            'the largest period start time that has been seen.');
+            `Skipping Period ${i+1} as its start time is smaller than ` +
+            'the largest period start time that has been seen, and start ' +
+            'time is unseen before');
         continue;
+      } else if (!this.seenPeriodIds_.includes(start)) {
+        this.seenPeriodIds_.push(start);
       }
 
       // Save maximum period start time if it is the last period

test/dash/dash_parser_manifest_unit.js

@@ -2198,7 +2198,8 @@ describe('DashParser Manifest', () => {
   it('skip periods that are earlier than max period start time', async () => {
     const sources = [
       buildManifestWithPeriodStartTime([5, 15]),
-      buildManifestWithPeriodStartTime([4, 15]), // simulate out-of-sync of -1s
+      buildManifestWithPeriodStartTime([6, 15]), // simulate out-of-sync of -1s
+      buildManifestWithPeriodStartTime([4, 15]), // simulate out-of-sync of +1s
     ];
     const segments = [];