Prevent Duplicate Subscriptions

[Abd-Standard]

Description
Ensure users cannot subscribe multiple times to the same notification channel.

Acceptance Criteria
Duplicate subscriptions are rejected.
Existing subscriptions remain unaffected.
Unit tests validate duplicate checks.

[rindicomfort]

@rindicomfort has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello Maintainer,

Please I’d love to take this issue on. I’m a smart contract and full-stack developer with experience across OnlyDust, Drips Waves

My stack includes Rust, Soroban, Cairo, Solidity, and TypeScript. I can deliver a clean, tested, and well-documented solution within 24 hours of assignment.

Happy to start immediately. Thank You

ℹ️ Repo Maintainers: To accept this application, review their application or assign @rindicomfort to this issue.

[coderolisa]

@coderolisa has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello maintainer let me work on this
please assign

ℹ️ Repo Maintainers: To accept this application, review their application or assign @coderolisa to this issue.

[drips-wave]

Congratulations, @coderolisa! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @coderolisa: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox!

@coderolisa's PR #558 was approved and merged by @Abd-Standard.

🏆 @coderolisa: You earned 40 FoxPoints for this contribution! Your current tier: Builder (1,302 total points). Track your full progress on GrantFox.

👏 Great work, @coderolisa! Keep contributing to Core-Foundry.