Bugfix for add to lesson

[Clarenceeey]

Fixes #300 partially addresses #299

Previously, when the same student is added to the lesson in the same command, it throws an exception.

Now, the student will be added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ	Complexity Δ
...ress/logic/commands/lesson/AddToLessonCommand.java	90.54% <100.00%> (+1.14%)	17.00 <0.00> (+5.00)

... and 3 files with indirect coverage changes

[notnotmax]

lgtm. Also, I believe this should be allowed during feature freeze because it is clearly a 'bug' i.e. does not conform to reasonable expected behavior due to an error in the code.