feat(qt): warn when sending to duplicate recipients

[UdjinM6]

Issue being fixed or feature implemented

Having duplicated recipients is ok on protocol level, we could be less restrictive in GUI.

Inspired by #7012

What was done?

Handle DuplicateAddress as a warning status instead of an error. Show a confirmation dialog when duplicates are detected, allowing the transaction to proceed if the user confirms.

How Has This Been Tested?

Run dash-qt, send a tx

Breaking Changes

n/a

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

[github-actions]

⚠️ Potential Merge Conflicts Detected

This PR has potential conflicts with the following open PRs:

• feat(qt): Add flag to allow duplicate recipient addresses in GUI #7012 - feat(qt): Add flag to allow duplicate recipient addresses in GUI

Please coordinate with the authors of these PRs to avoid merge conflicts.

[coderabbitai]

Walkthrough

The PR changes duplicate-address handling during send operations. In walletmodel.cpp the duplicate-address check is moved to occur after fee computation/absurd-fee validation rather than before balance/fee checks. In sendcoinsdialog.cpp, when a DuplicateAddress prepare status is returned the UI now shows a confirmation dialog and only aborts the send if the user declines; other non-OK prepare statuses still abort immediately.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20–30 minutes

• Review walletmodel.cpp: confirm reordering of duplicate-address check doesn't alter error precedence or expose fee/balance checks to unexpected inputs.
• Review sendcoinsdialog.cpp: verify the confirmation dialog blocks and the abort/continue paths are correct and thread-safe.
• Ensure processSendCoinsReturn change (removing user-facing DuplicateAddress message) aligns with UX requirements and that compiler-silencing placement is correct.

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 'feat(qt): warn when sending to duplicate recipients' clearly and concisely describes the main change: converting duplicate recipient handling from an error to a warning in the Qt GUI.
Description check	✅ Passed	The description is related to the changeset, explaining the rationale (protocol allows duplicates), what was done (warning dialog instead of error), and how it was tested (run dash-qt and send tx).

✨ 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 2f2a3fc and e14aaa0.

📒 Files selected for processing (2)

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

🚧 Files skipped from review as they are similar to previous changes (2)

• src/qt/sendcoinsdialog.cpp
• src/qt/walletmodel.cpp

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (7)

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• GitHub Check: x86_64-w64-mingw32 / Build depends
• 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

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[PastaPastaPasta]

please rebase to fix: https://github.com/dashpay/dash/actions/runs/19746402134/job/56581519530?pr=7015

[nmarley]

LGTM 👍. I like it better than #7012

[PastaPastaPasta]

utACK e14aaa0