diff --git a/change/@griffel-core/warn-duplicate-rehydration-styles-20260620155350.json b/change/@griffel-core/warn-duplicate-rehydration-styles-20260620155350.json
new file mode 100644
index 000000000..3f1bfe025
--- /dev/null
+++ b/change/@griffel-core/warn-duplicate-rehydration-styles-20260620155350.json
@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "Warn in development when duplicate CSS rules are found during rehydration — a signal that server-rendered styles were flushed into the HTML more than once (e.g. the Next.js App Router calling renderToStyleElements on every flush without clearing the renderer)",
+  "packageName": "@griffel/core",
+  "email": "vaclav.dvorak@ysoft.com",
+  "dependentChangeType": "patch"
+}
diff --git a/packages/core/src/renderer/rehydrateRendererCache.test.ts b/packages/core/src/renderer/rehydrateRendererCache.test.ts
index 37855a730..570192b63 100644
--- a/packages/core/src/renderer/rehydrateRendererCache.test.ts
+++ b/packages/core/src/renderer/rehydrateRendererCache.test.ts
@@ -1,4 +1,4 @@
-import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { DATA_BUCKET_ATTR, DATA_PRIORITY_ATTR } from '../constants.js';
 import type { GriffelRenderer } from '../types.js';
 import { createDOMRenderer } from './createDOMRenderer.js';
@@ -105,4 +105,52 @@ describe('rehydrateRendererCache', () => {
       }
     `);
   });
+
+  it('warns in development when the same rule is rehydrated from multiple <style> elements', () => {
+    process.env.NODE_ENV = 'development';
+    const consoleError = vi.spyOn(console, 'error').mockImplementation(() => undefined);
+
+    // The same rule emitted into two server-rendered <style> elements is the signature of
+    // styles being flushed into the HTML more than once — e.g. a Next.js App Router setup that
+    // calls renderToStyleElements() on every flush without clearing the renderer in between.
+    for (let i = 0; i < 2; i++) {
+      const styleElement = document.createElement('style');
+      styleElement.setAttribute(DATA_BUCKET_ATTR, 'd');
+      styleElement.setAttribute(DATA_PRIORITY_ATTR, '0');
+      styleElement.textContent = '.foo { color: red; }';
+      document.head.appendChild(styleElement);
+    }
+
+    rehydrateRendererCache(renderer, document);
+
+    expect(consoleError).toHaveBeenCalledTimes(1);
+    expect(consoleError.mock.calls[0][0]).toContain('duplicate CSS rule');
+
+    consoleError.mockRestore();
+  });
+
+  it('does not warn when buckets repeat with different rules (correct streamed delta)', () => {
+    process.env.NODE_ENV = 'development';
+    const consoleError = vi.spyOn(console, 'error').mockImplementation(() => undefined);
+
+    // Same bucket + priority across two elements, but different rules — this is what correct
+    // per-flush delta flushing looks like, and it must NOT warn.
+    const first = document.createElement('style');
+    first.setAttribute(DATA_BUCKET_ATTR, 'd');
+    first.setAttribute(DATA_PRIORITY_ATTR, '0');
+    first.textContent = '.foo { color: red; }';
+    document.head.appendChild(first);
+
+    const second = document.createElement('style');
+    second.setAttribute(DATA_BUCKET_ATTR, 'd');
+    second.setAttribute(DATA_PRIORITY_ATTR, '0');
+    second.textContent = '.bar { color: blue; }';
+    document.head.appendChild(second);
+
+    rehydrateRendererCache(renderer, document);
+
+    expect(consoleError).not.toHaveBeenCalled();
+
+    consoleError.mockRestore();
+  });
 });
diff --git a/packages/core/src/renderer/rehydrateRendererCache.ts b/packages/core/src/renderer/rehydrateRendererCache.ts
index 7994cc9c8..89af2412e 100644
--- a/packages/core/src/renderer/rehydrateRendererCache.ts
+++ b/packages/core/src/renderer/rehydrateRendererCache.ts
@@ -29,6 +29,29 @@ export function rehydrateRendererCache(
   if (target) {
     const styleElements = target.querySelectorAll<HTMLStyleElement>('[data-make-styles-bucket]');
 
+    // Griffel emits each CSS rule into the document exactly once. In development we track
+    // the rules seen while rehydrating, so we can warn if the same rule appears in more than
+    // one server-rendered <style> element — a strong signal that styles were flushed into the
+    // HTML multiple times (see the warning emitted after the loop).
+    const seenRules: Set<string> | undefined = process.env.NODE_ENV !== 'production' ? new Set() : undefined;
+    let duplicateRuleCount = 0;
+
+    const cacheRule = (cssRule: string, bucketName: StyleBucketName) => {
+      if (seenRules) {
+        if (seenRules.has(cssRule)) {
+          duplicateRuleCount++;
+        } else {
+          seenRules.add(cssRule);
+        }
+      }
+
+      renderer.insertionCache[cssRule] = bucketName;
+
+      if (process.env.NODE_ENV !== 'production' && isDevToolsEnabled) {
+        debugData.addCSSRule(cssRule);
+      }
+    };
+
     styleElements.forEach(styleElement => {
       const bucketName = styleElement.dataset['makeStylesBucket'] as StyleBucketName;
       const stylesheetKey = getStyleSheetKeyFromElement(styleElement);
@@ -47,11 +70,7 @@ export function rehydrateRendererCache(
         while ((match = regex.exec(textContent))) {
           const [cssRule] = match;
 
-          renderer.insertionCache[cssRule] = bucketName;
-
-          if (process.env.NODE_ENV !== 'production' && isDevToolsEnabled) {
-            debugData.addCSSRule(cssRule);
-          }
+          cacheRule(cssRule, bucketName);
         }
       } else {
         // @scope rules can appear in any bucket, so extract them first to prevent
@@ -60,25 +79,40 @@ export function rehydrateRendererCache(
         while ((match = SCOPE_HYDRATOR.exec(textContent))) {
           const [cssRule] = match;
 
-          renderer.insertionCache[cssRule] = bucketName;
+          cacheRule(cssRule, bucketName);
           textContent = textContent.replace(cssRule, '');
-
-          if (process.env.NODE_ENV !== 'production' && isDevToolsEnabled) {
-            debugData.addCSSRule(cssRule);
-          }
         }
 
         // eslint-disable-next-line no-cond-assign
         while ((match = STYLES_HYDRATOR.exec(textContent))) {
           const [cssRule] = match;
 
-          renderer.insertionCache[cssRule] = bucketName;
-
-          if (process.env.NODE_ENV !== 'production' && isDevToolsEnabled) {
-            debugData.addCSSRule(cssRule);
-          }
+          cacheRule(cssRule, bucketName);
         }
       }
     });
+
+    if (process.env.NODE_ENV !== 'production' && duplicateRuleCount > 0) {
+      // eslint-disable-next-line no-console
+      console.error(
+        [
+          '@griffel/core:',
+          `Found ${duplicateRuleCount} duplicate CSS rule(s) while rehydrating server-rendered styles.`,
+          'The same styles were flushed into the HTML more than once.',
+          '\n\n',
+          'In the Next.js App Router this happens when renderToStyleElements() is called on every',
+          'useServerInsertedHTML flush without clearing the renderer in between: each flush re-emits',
+          'the full stylesheet and the extra copies are streamed into <body>. Those stale copies can',
+          'override styles inserted after a client-side navigation, making makeStyles() overrides lose',
+          'to their makeResetStyles() base.',
+          '\n\n',
+          'Clear the renderer after each flush:',
+          '\n\n',
+          'useServerInsertedHTML(() => {\n  const styles = renderToStyleElements(renderer);\n  renderer.stylesheets = {};\n  return styles;\n});',
+          '\n\n',
+          'See https://griffel.js.org/react/guides/ssr-usage for details.',
+        ].join(' '),
+      );
+    }
   }
 }