Test `assert_unique_recipients` with exactly two identical addresses returns error

[Kingsman-99]

Description

The existing tests in contracts/split/src/validation.rs cover a 3-element list with one duplicate. A simpler minimal test with exactly two identical addresses would more directly target the core duplicate-detection logic and makes the test suite easier to read.

Acceptance Criteria

• Add a test named two_identical_addresses_returns_duplicate_error
• Build a Vec<Address> with two copies of the same address
• Assert assert_unique_recipients returns Err(ContractError::DuplicateRecipient)
• cargo test passes

Context

• Target file: contracts/split/src/validation.rs

[RemmyAcee]

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I've checked the requirements and am confident I can resolve this efficiently. Happy to take it on.

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[goldemaverick-ui]

@goldemaverick-ui has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello maintainer, would love to take ownership of this issue implement a good fix and open a clean ready to merge PR..

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[grantfox-oss]

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

@goldemaverick-ui's PR #647 was approved and merged by @Kingsman-99.

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This issue has been completed by @goldemaverick-ui as part of the Stellar Wave Program's 8th Wave 🥳

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