Duplicate expense - Page become unresponsive when duplicating bulk expenses

[jponikarchuk]

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Version Number: v9.3.58-0
Reproducible in staging?: Yes
Reproducible in production?: N/A - new feature, doesn't exist in prod
If this was caught during regression testing, add the test name, ID and link from BrowserStack: #84657
Email or phone of affected tester (no customers): testpayment935@gmail.com
Issue reported by: Applause Internal Team
Bug source: Exploratory - Significant User Experience Deterioration
Device used: Windows 11 pro /Chrome
App Component: Money Requests

Action Performed:

1. Navigate to staging.new.expensify.com
2. Sign in with 200+ expense account ( you can duplicate until you have that amount)
3. Go to reports and select all expense and duplicate

Expected Result:

The expense will be duplicated without an issue

Actual Result:

The page become unresponsive when duplicating a large amount of expense

Workaround:

Unknown

Platforms:

• Android: App
• Android: mWeb Chrome
• iOS: App
• iOS: mWeb Safari
• iOS: mWeb Chrome
• Windows: Chrome
• MacOS: Chrome / Safari

Screenshots/Videos

Video.mp4

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[melvin-bot]

You have been assigned to this deploy blocker because you recently merged this PR: #84657

[melvin-bot]

💬 A slack conversation has been started in #expensify-open-source

[github-actions]

👋 Friendly reminder that deploy blockers are time-sensitive ⏱ issues! Check out the open `StagingDeployCash` deploy checklist to see the list of PRs included in this release, then work quickly to do one of the following:

1. Identify the pull request that introduced this issue and revert it.
2. Find someone who can quickly fix the issue.
3. Fix the issue yourself.

[OSBotify]

Investigation Summary

Classification: Frontend bug
Causing PR: #84657 - "[Duplicate Expense] Allow bulk duplication." by @Krishna2323 (High confidence)
Related Issues: #84281

Recommendation: REVERT

PR #84657 introduced a synchronous loop that calls duplicateExpenseTransaction once per selected transaction with no batching or throttling. With 200+ expenses selected, this triggers hundreds of back-to-back Onyx writes and API preparations on the main JS thread, blocking the event loop and making the page unresponsive.

Assigned: @Krishna2323 (PR author), @JS00001 @situchan (approving reviewers)
Labels: No label changes needed — DeployBlockerCash is correct (frontend App PR caused the issue; this is staging-only and does not exist in production)

Detailed Analysis

Evidence

• The melvin-bot already identified PR [Duplicate Expense] Allow bulk duplication.  #84657 as the causing PR in the first comment
• PR [Duplicate Expense] Allow bulk duplication.  #84657 is listed as unchecked in the current StagingDeployCash checklist (issue Deploy Checklist: New Expensify 2026-04-10 #87562), confirming it is part of the v9.3.58-0 staging release
• The bug is reproducible on staging and not applicable to production since this is a new feature
• The issue was reported against version v9.3.58-0, which matches the current staging deploy

Verification

• Affected code: src/libs/actions/IOU/Duplicate.ts — specifically the bulkDuplicateExpenses function
• Confirmation: PR [Duplicate Expense] Allow bulk duplication.  #84657 adds src/libs/actions/IOU/Duplicate.ts, src/hooks/useBulkDuplicateAction.ts, and src/components/Search/BulkDuplicateHandler.tsx — all directly on the path of the reported bug

Root Cause

bulkDuplicateExpenses in Duplicate.ts (lines 960-1034) runs a synchronous for loop over all selected transactions, calling duplicateExpenseTransaction for each one. Each call performs complex work including building optimistic Onyx updates, generating new report/transaction IDs, and preparing API parameters. With 200+ transactions this executes entirely on the main JS thread without yielding, causing a browser hang. There is no batching, chunking, or async deferral between iterations.