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Turbopack Dev Duplicates Shared CSS Module Chunk After Client Navigation, Breaking Cascade Order

In Next.js dev with Turbopack and Nx externalDir, a shared component's CSS module is emitted as a standalone chunk. On client navigation to a route that reuses the component, Turbopack appends a second copy of the stylesheet to <head> and does not remove it on Back, causing the later duplicate to override equal-specificity styles and break cascade order.

highConfidence 72%Next.js Affected V16.2.3 Affected V16.2.9 Affected V16.3.0-Canary.58

Origin Analysis

Turbopack's development client runtime does not deduplicate stylesheet link tags by href when the same CSS chunk is loaded by multiple route chunks. On navigation back, the previously appended duplicate link remains and appears after the original chunk that contains the user's override, so the duplicate's later .kindPrimary wins over the earlier .featureButton.

1. Clone https://github.com/ilchenkoArtem/turbopack-css-order-repro 2. npm install && npm run dev (runs nx serve web → next dev --turbopack) 3. Open home page; confirm the 'Show more' button is transparent. 4. Click the button to navigate to /catalog. 5. Press browser Back. 6. Observe the button becomes orange, indicating duplicate CSS chunk placed after original.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

Removing duplicate link tags is safe for identical CSS module chunks, but if a stylesheet is intentionally loaded multiple times with different media, crossorigin, or other attributes, this dedupe could drop a needed variant. The fix is dev-only and should be removed once the upstream Turbopack fix lands; if the duplicate appears first in DOM order (unlikely), removing later occurrences would be wrong. For production, rely on the corrected build output; rollback by deleting this component and verifying cascade order.