fix(website/ta): Prevent duplicate TA lesson in reducer

[leslieyip02]

Context

Currently, the TA module reducer doesn't check for duplicates when adding a new lesson.

Mentioned in #3930 under this comment.

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
nusmods-export	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jan 4, 2025 1:05pm

[vercel]

@leslieyip02 is attempting to deploy a commit to the modsbot's projects Team on Vercel.

A member of the Team first needs to authorize it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.03%. Comparing base (988c6fd) to head (7205f8e).
Report is 67 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3940      +/-   ##
==========================================
+ Coverage   54.52%   55.03%   +0.51%     
==========================================
  Files         274      276       +2     
  Lines        6076     6301     +225     
  Branches     1455     1534      +79     
==========================================
+ Hits         3313     3468     +155     
- Misses       2763     2833      +70     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.