Add contract-level duplicate payment detection and rejection

[Kingsman-99]

Description

Identical payments (same payer, same amount, same ledger) can be submitted twice due to network retries, resulting in double-charging. This issue adds a payment fingerprint registry that detects and rejects duplicate submissions within a configurable ledger window.

Technical Context

Compute a payment fingerprint hash = sha256(invoice_id || payer || amount || ledger_sequence) using env.crypto().sha256() in lib.rs. Store fingerprint_key(hash) -> bool with a TTL of DUPLICATE_WINDOW_LEDGERS. Before processing pay_invoice, check for an existing fingerprint entry; if present, panic with DuplicatePayment. After processing, store the fingerprint.

Acceptance Criteria

• Payment fingerprint is computed from invoice_id, payer, amount, and current ledger sequence
• Duplicate submission within DUPLICATE_WINDOW_LEDGERS panics with DuplicatePayment
• Fingerprint storage uses Soroban temporary storage with correct TTL so old entries expire automatically
• Different amounts from the same payer in the same ledger are NOT considered duplicates
• DUPLICATE_WINDOW_LEDGERS is admin-configurable; default 5 ledgers
• Tests: identical call twice fails, same payer different amount passes, after window expiry same call succeeds
• All CI checks (cargo test, cargo clippy, cargo fmt) pass and the branch has no merge conflicts

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[adeboladee]

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

Hello Maintainer, Please I would like to work on this issue

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[NeduTheDev1]

@NeduTheDev1 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Please assign to me maintainer I can handle with no issues

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[Anadudev]

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

Hi, maintainer, I would like to work on this issue.

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[octobersmvp]

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

Hi @Stellar-split! I'll implement duplicate payment detection in lib.rs by computing a SHA256 payment fingerprint and persisting it in temporary storage with a configurable DUPLICATE_WINDOW_LEDGERS TTL. I'll also add comprehensive tests covering duplicate rejections, different-amount passes, and window expiration.

Please assign this to me!

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[williamsokperez-arch]

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

i would like to work on this issue

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[drips-wave]

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

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@williamsokperez-arch's PR #459 was approved and merged by @Kingsman-99.

🏆 @williamsokperez-arch: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (344 total points). Track your full progress on GrantFox.

👏 Great work, @williamsokperez-arch! Keep contributing to Stellar-split.

[drips-wave]

This issue has been completed by @williamsokperez-arch as part of the Stellar Wave Program's 7th Wave 🥳

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