diff --git a/packages/core/src/extensions/mermaid.test.ts b/packages/core/src/extensions/mermaid.test.ts
new file mode 100644
index 0000000000..a950e1e77e
--- /dev/null
+++ b/packages/core/src/extensions/mermaid.test.ts
@@ -0,0 +1,47 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { initRenderer } from '../renderer/renderer-impl'
+import { renderMarkdown } from '../utils/markdownHelpers'
+
+const mermaid = vi.hoisted(() => {
+  let release: (() => void) | undefined
+  const gate = new Promise<void>((resolve) => {
+    release = resolve
+  })
+  return {
+    initialize: vi.fn(),
+    render: vi.fn(async (id: string) => {
+      await gate
+      return { svg: `<svg id="${id}" viewBox="0 0 10 10"></svg>` }
+    }),
+    release: () => release!(),
+  }
+})
+
+vi.mock(`mermaid`, () => ({
+  default: { initialize: mermaid.initialize, render: mermaid.render },
+}))
+
+function render(md: string): string {
+  return renderMarkdown(md, initRenderer({ themeMode: `light` })).html
+}
+
+describe(`mermaid extension`, () => {
+  afterEach(() => {
+    document.body.innerHTML = ``
+  })
+
+  it(`shares a pending render so re-renders cannot clash on the element id`, async () => {
+    const markdown = `\`\`\`mermaid\ngraph TD\n  A --> B\n\`\`\``
+    const host = document.createElement(`div`)
+    document.body.append(host)
+
+    host.innerHTML = render(markdown)
+    host.innerHTML = render(markdown)
+    await vi.waitFor(() => expect(mermaid.render).toHaveBeenCalled())
+    mermaid.release()
+
+    await vi.waitFor(() => expect(host.querySelector(`.mermaid-diagram svg`)).not.toBeNull())
+    expect(mermaid.render).toHaveBeenCalledTimes(1)
+    expect(render(markdown)).toContain(`viewBox="0 0 10 10"`)
+  })
+})
diff --git a/packages/core/src/extensions/mermaid.ts b/packages/core/src/extensions/mermaid.ts
index a46b0ed93a..6cbcb9592a 100644
--- a/packages/core/src/extensions/mermaid.ts
+++ b/packages/core/src/extensions/mermaid.ts
@@ -54,17 +54,29 @@ function buildCacheKey(code: string, themeMode?: DiagramThemeMode): string {
 
 const svgCache = createSVGCache(50)
 
+// A re-render while a diagram is still rendering (e.g. on `md:mathjax-ready`) would
+// start a second mermaid.render with the same element id; the two runs then measure
+// each other's temporary elements and cache an SVG without viewBox or size.
+const pendingRenders = new Map<string, Promise<string>>()
+
 async function renderMermaidSvg(code: string, themeMode?: DiagramThemeMode): Promise<string> {
   const cacheKey = buildCacheKey(code, themeMode)
   const cached = svgCache.get(cacheKey)
   if (cached)
     return cached
 
-  const mermaid = await getMermaid()
-  mermaid.initialize(getMermaidThemeConfig(themeMode))
-  const result = await mermaid.render(`mermaid-svg-${cacheKey}`, code)
-  svgCache.set(cacheKey, result.svg)
-  return result.svg
+  let pending = pendingRenders.get(cacheKey)
+  if (!pending) {
+    pending = (async () => {
+      const mermaid = await getMermaid()
+      mermaid.initialize(getMermaidThemeConfig(themeMode))
+      const result = await mermaid.render(`mermaid-svg-${cacheKey}`, code)
+      svgCache.set(cacheKey, result.svg)
+      return result.svg
+    })().finally(() => pendingRenders.delete(cacheKey))
+    pendingRenders.set(cacheKey, pending)
+  }
+  return pending
 }
 
 function renderMermaid(id: string, code: string, cacheKey: string, themeMode?: DiagramThemeMode) {