design(import): reject duplicate effective CSV headers instead of last-wins

[yinlianghui]

Baseline and classification

• Verified at 98874656ffc50ce1531af52346228ffcdda73fba.
• Design/validation gap, not a confirmed perf/reliability: seed + data-import bypass the engine's bulk-insert path → N round-trips, N summary recomputes, no transient retry #2678/perf(seed,import): route bulk writes through the engine's batch insert path #2680 regression.
• The current CSV parser arrived after perf(seed,import): route bulk writes through the engine's batch insert path #2680 in feat(auth): 管理员直建用户 + 手机号登录 + sys_user 批量导入（#2766 V1/V1.5/V2） #2771 (2a71f4825). No exact duplicate issue was found.

Minimal reproduction

Import this CSV while mapping both title columns to title:

ID,标题,标题
dup,first,second

Both executions succeed and persist title=second; the first value is silently discarded.

The same ambiguity can occur when distinct source headers map to the same destination field.

Decision needed / current behavior

• Safer expected behavior: reject duplicate effective destination headers/keys before writes, with a request-level or row-independent mapping error.
• Actual: parseCsvToRows assigns object keys in order, so later values silently overwrite earlier ones.

Impact

Malformed files or ambiguous mapping can cause silent data loss while the import summary reports success.

Evidence and exclusions

• Source: packages/rest/src/import-prepare.ts, CSV header mapping/object assignment.
• Quoted commas, BOM, CRLF, escaped quotes, and ordinary mappings passed QA.
• Because no public duplicate-header contract was found, this issue requests an explicit validation decision rather than labeling existing behavior a regression.

Required regression tests

• Duplicate raw headers.
• Two distinct raw headers mapped to one destination field.
• Error occurs before any row is written and identifies the conflicting columns/field.

[yinlianghui]

独立重现审计补充（2026-07-11）

Issue 原文只给出了重复 raw header 的具体 CSV；这里补齐“不同源列映射到同一目标字段”的精确输入。固定 baseline 98874656ffc50ce1531af52346228ffcdda73fba，对具有 id/title 字段的全新对象，通过真实 POST /api/v1/data/:object/import route 分别运行一次：

1. 重复 raw header：CSV ID,标题,标题\ndup,first,second，mapping {"ID":"id","标题":"title"}。
2. 不同源列映射同目标：CSV ID,标题A,标题B\ndup,first,second，mapping {"ID":"id","标题A":"title","标题B":"title"}。

两次均 HTTP 200，summary 均为 total=1, ok=1, errors=0, created=1，持久化结果均为 id=dup, title=second；route/harness 进程 exit 0。也就是两种歧义都在任何写前校验之前静默 last-wins。分类仍是 design/validation gap，不是 #2678/#2680 回归。

[yinlianghui]

暂时关闭，等待人工逐项复核。当前 QA 复现证据保留；本次关闭不表示问题已修复、无效或重复。人工确认后将按需重新打开。