fix(website/ta): Prevent duplicate TA lessons in timetable (#3942)

Co-authored-by: Zhao Wei Liew <[email protected]>

Author: leslieyip02

website/src/views/timetable/TimetableContent.tsx

@@ -369,6 +369,13 @@ class TimetableContent extends React.Component<Props, State> {
           title: module.title,
         };
 
+        // Prevent multiple versions of the same lesson
+        if (
+          timetableLessons.some((curLesson) => areLessonsSameClass(modifiableLesson, curLesson))
+        ) {
+          return;
+        }
+
         // All lessons added within this block are options to be added in the timetable
         // Except for the activeLesson
         modifiableLesson.isOptionInTimetable = true;