Bug in detecting duplicate telegram

[zlllllr]

Description: According to the UG, each person must have a unique Telegram and telegram is case-insensitive. Hence, adding a person with telegram with @aaaaa and @aaaaa should be considered as a duplicate person

Steps to reproduce:
add n/test t/@aaaaa
add n/test t/@aaaaa

Expected: The app disallow the second addition as the second person has duplicate telegram

Actual: The second person is allowed to be added

Screenshots:

[nus-se-script]

Your response not required for this bug as the team has accepted the bug as it is.