diff --git a/web/e2e/copy-dialog.spec.ts b/web/e2e/copy-dialog.spec.ts
index 829719ae2..bdde381b3 100644
--- a/web/e2e/copy-dialog.spec.ts
+++ b/web/e2e/copy-dialog.spec.ts
@@ -2,6 +2,7 @@ import { test, expect } from './fixtures';
 import { browserLogin, seedDoc } from './lib/collab-helpers';
 import type { APIRequestContext, Locator, Page } from '@playwright/test';
 import type { SuiteFixture } from './fixtures';
+import { COPY_PREFLIGHT_DEBOUNCE_MS } from '../src/lib/items/copyPreflightTiming';
 
 /**
  * Cross-workspace copy / move dialog (PLAN-2373 / TASK-2355).
@@ -601,7 +602,13 @@ test.describe('cross-workspace copy dialog (PLAN-2373 / TASK-2355)', () => {
 
 	});
 
-	test('override edits collapse to one in-flight preflight plus one trailing run', async ({
+	// Two spacings (BUG-3151). Picks faster than the debounce
+	// (COPY_PREFLIGHT_DEBOUNCE_MS, CopyItemDialog's runner) collapse in
+	// the timer; picks SLOWER than it each fire a run, which queues the trailing
+	// run while #2 is held — and the last pick's own timer is then still pending
+	// when that trailing run starts. It used to fire afterwards with an identical
+	// body (4 requests), which only full-suite load made slow enough to happen.
+	for (const gap of [0, COPY_PREFLIGHT_DEBOUNCE_MS + 100]) test(`override edits collapse to one in-flight preflight plus one trailing run (picks ${gap}ms apart)`, async ({
 		page,
 		fixture,
 		request,
@@ -657,11 +664,16 @@ test.describe('cross-workspace copy dialog (PLAN-2373 / TASK-2355)', () => {
 		await expect.poll(() => started).toBe(2);
 
 		await pick(1);
+		if (gap) await page.waitForTimeout(gap);
 		await pick(2);
 		// Single-flight: neither pick issued a request of its own.
 		expect(started).toBe(2);
 		releaseSecond();
 		await expect(dialog.getByRole('button', { name: 'Copy', exact: true })).toBeEnabled();
+		// Outlast any debounce still pending, so a late duplicate is counted
+		// here rather than after the assertion (BUG-3151: this count used to be
+		// read before the stray timer fired, except under load).
+		await page.waitForTimeout(2 * COPY_PREFLIGHT_DEBOUNCE_MS + 100);
 		// …and both were served by ONE trailing run, not one request each.
 		expect(started).toBe(3);
 	});
diff --git a/web/src/lib/components/items/CopyItemDialog.svelte b/web/src/lib/components/items/CopyItemDialog.svelte
index a27e6aeb1..4f636565c 100644
--- a/web/src/lib/components/items/CopyItemDialog.svelte
+++ b/web/src/lib/components/items/CopyItemDialog.svelte
@@ -43,6 +43,7 @@ their own recovery paths; swallowing those into outcome-unknown would send the
 user hunting for an item that provably does not exist.
 -->
 <script lang="ts">
+	import { COPY_PREFLIGHT_DEBOUNCE_MS } from '$lib/items/copyPreflightTiming';
 	import { tick, untrack } from 'svelte';
 	import Modal from '$lib/components/common/Modal.svelte';
 	import FieldEditor from '$lib/components/fields/FieldEditor.svelte';
@@ -120,7 +121,7 @@ user hunting for an item that provably does not exist.
 	// So: a debounce, PLUS single-flight trailing coalescing (round-12 fold-in).
 	// At most one preflight is in flight at any moment; edits that settle while
 	// one is running collapse into exactly ONE trailing run.
-	const PREFLIGHT_DEBOUNCE_MS = 250;
+	const PREFLIGHT_DEBOUNCE_MS = COPY_PREFLIGHT_DEBOUNCE_MS;
 
 	/**
 	 * Types the dialog can safely collect a value for — the set `FieldEditor`
@@ -618,6 +619,14 @@ user hunting for an item that provably does not exist.
 		// arrives while one is running sets the trailing flag, and exactly one
 		// trailing run fires when the current one settles — so a burst of edits
 		// across many inputs collapses to (in-flight + 1) rather than N.
+		// A run reads the CURRENT state when it starts (buildRequest), so it
+		// answers every edit made before that moment — including one whose
+		// debounce timer is still pending. Left pending, that timer fires after
+		// and sends the same body again: a duplicate preview whenever edits are
+		// spaced wider than the debounce while one is in flight (BUG-3151, the
+		// full-suite-only failure of copy-dialog.spec.ts' coalescing test).
+		clearTimeout(debounceTimer);
+		debounceTimer = undefined;
 		if (preflightInFlight) {
 			trailingQueued = true;
 			return;
diff --git a/web/src/lib/items/copyPreflightTiming.ts b/web/src/lib/items/copyPreflightTiming.ts
new file mode 100644
index 000000000..c8928e0c9
--- /dev/null
+++ b/web/src/lib/items/copyPreflightTiming.ts
@@ -0,0 +1,4 @@
+// The copy dialog's preview debounce (CopyItemDialog.svelte), in a plain module
+// so the e2e spec that measures the runner can derive its timings from it
+// (BUG-3151): a change here retunes the spec instead of silently weakening it.
+export const COPY_PREFLIGHT_DEBOUNCE_MS = 250;