Prevent Duplicate Applications

[nensii21]

Description

A user should not be able to apply multiple times to the same project.

Acceptance Criteria
Validation added
Proper HTTP response
Existing tests updated

[Ayush-0918]

Hi @nensii21,

I'd like to work on this issue under ECSoC 2026.

I'll add backend validation to prevent duplicate applications, return a proper HTTP response for duplicate requests, and update the existing tests accordingly.

Please assign this issue to me if it's available.

Thank you!

[Ayush-0918]

Hi @nensii21, I've pushed the requested changes. Please let me know if anything else needs to be updated. Thanks!