feat(qt): Add flag to allow duplicate recipient addresses in GUI

[nmarley]

This enables users of the Qt wallet to send to the same address in a single transaction. This is useful for setting up multiple MN collateral transactions, but probably only for advanced users and developers.

Usage: ./qt/dash-qt -allowduplicaterecipients

I'm ok if there's no desire to merge this. I just thought it was a nice feature to send all collateral in a single TX instead of having to send multiple. I presume this is mostly just useful for testnet, as a hardware wallet would be preferred for mainnet, but it may also work to create an unsigned TX which could be signed by a HW wallet w/the right UI.

[github-actions]

✅ No Merge Conflicts Detected

This PR currently has no conflicts with other open PRs.

[coderabbitai]

Walkthrough

Adds a GUI command-line option -allowduplicaterecipients (debug/testing-only) to src/qt/bitcoin.cpp and a corresponding boolean fAllowDuplicateDestAddr in src/qt/walletmodel.cpp. Transaction preparation logic in WalletModel conditionally skips the duplicate-recipient-address validation when this flag is true. Default behavior (flag false) and other validation, balance checks, and transaction construction remain unchanged.

Sequence Diagram

sequenceDiagram
    participant User
    participant GUI as "GUI Layer\n(src/qt/bitcoin.cpp)"
    participant WalletModel as "WalletModel\n(src/qt/walletmodel.cpp)"
    participant TxValidation as "Tx Validation\n(duplicate-address check)"

    User->>GUI: Launch app (may set -allowduplicaterecipients)
    GUI->>WalletModel: Initialize (passes fAllowDuplicateDestAddr)
    User->>GUI: Create transaction with recipients (possible duplicates)
    GUI->>WalletModel: PrepareTransaction request
    WalletModel->>TxValidation: Check for duplicate recipient addresses
    alt fAllowDuplicateDestAddr == true
        TxValidation-->>WalletModel: Duplicate check skipped (allowed)
    else fAllowDuplicateDestAddr == false
        TxValidation->>TxValidation: Validate no duplicate addresses
        TxValidation-->>WalletModel: Return validation result
    end
    WalletModel-->>GUI: Return prepared transaction / error

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Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

• Changes limited to two Qt files and involve a single conditional flag.
• Low logic density and consistent pattern.

Areas requiring attention:

• Ensure the CLI option is registered with correct flags (ALLOW_ANY | DEBUG_ONLY) and default value.
• Verify fAllowDuplicateDestAddr is properly read and used in all duplicate-address checks in WalletModel.
• Confirm no additional places enforce duplicate-address constraints outside the modified check.

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely describes the main change: adding a flag to allow duplicate recipient addresses in the Qt GUI wallet.
Description check	✅ Passed	The description clearly explains the feature's purpose (sending multiple MN collateral in one transaction), usage example, target audience (advanced users/developers), and implementation considerations.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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📜 Recent review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between f51e0d0 and 1808006.

📒 Files selected for processing (2)

• src/qt/bitcoin.cpp (1 hunks)
• src/qt/walletmodel.cpp (2 hunks)

🚧 Files skipped from review as they are similar to previous changes (1)

• src/qt/walletmodel.cpp

🧰 Additional context used

📓 Path-based instructions (2)

src/**/*.{cpp,h,hpp,cc}

📄 CodeRabbit inference engine (CLAUDE.md)

Dash Core implementation must be written in C++20, requiring at least Clang 16 or GCC 11.1

Files:

• src/qt/bitcoin.cpp

src/qt/**/*.{cpp,h}

📄 CodeRabbit inference engine (CLAUDE.md)

GUI implementation in src/qt/ must use Qt 5

Files:

• src/qt/bitcoin.cpp

🧠 Learnings (1)

📓 Common learnings

Learnt from: kwvg
Repo: dashpay/dash PR: 6543
File: src/wallet/receive.cpp:240-251
Timestamp: 2025-02-06T14:34:30.466Z
Learning: Pull request #6543 is focused on move-only changes and refactoring, specifically backporting from Bitcoin. Behavior changes should be proposed in separate PRs.

Learnt from: kwvg
Repo: dashpay/dash PR: 6761
File: src/chainlock/signing.cpp:247-250
Timestamp: 2025-07-29T14:32:48.369Z
Learning: In PR #6761, kwvg acknowledged a null pointer check issue in ChainLockSigner::Cleanup() method but deferred it to follow-up, consistent with the pattern of avoiding scope creep in refactoring PRs.

🧬 Code graph analysis (1)

src/qt/bitcoin.cpp (1)

src/wallet/init.cpp (1)

• argsman (44-44)

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• GitHub Check: x86_64-pc-linux-gnu_nowallet / Build depends
• GitHub Check: arm-linux-gnueabihf / Build depends
• GitHub Check: x86_64-pc-linux-gnu / Build depends

🔇 Additional comments (1)

src/qt/bitcoin.cpp (1)

501-501: LGTM! Appropriate use of DEBUG_ONLY flag for this testing feature.

The argument registration is correctly structured and the use of ArgsManager::DEBUG_ONLY is appropriate given that duplicate recipient addresses can be confusing for regular users. The "For testing purposes only" description clearly communicates the intent, and the default disabled state (0) provides a safe baseline.

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[nmarley]

Force-pushed w/a GPG-signed commit, no changes to code.