diff --git a/server/services/previewDeploy.js b/server/services/previewDeploy.js
index 46b2b88..69a4592 100644
--- a/server/services/previewDeploy.js
+++ b/server/services/previewDeploy.js
@@ -4,6 +4,7 @@ import fs from 'node:fs/promises'
 import path from 'node:path'
 import { config } from '../config.js'
 import { listDocuments, Query } from './appwriteAdmin.js'
+import { withLocks } from './keyedLock.js'
 
 const execFileAsync = promisify(execFile)
 
@@ -428,8 +429,17 @@ export async function runDeploy(repoFullName, branch, previewConfig) {
   }
 
   const subdomain = previewConfig.subdomain || repoFullName.split('/').pop()
-  const cloneUrl = `${config.gitea.baseUrl}/${repoFullName}.git`
   const workDir = previewWorkDir(repoFullName)
+  // Deploys desselben Repos oder derselben Subdomain nacheinander: Projektaenderung und
+  // Gitea-Webhook kamen gleichzeitig, beide git clone liefen ins selbe workDir und
+  // brachen ab (Kirschner Gebaeudeservice, 30.09.2026).
+  return withLocks([`preview-work:${workDir}`, `preview-site:${subdomain}`], () =>
+    deployLocked(repoFullName, branch, previewConfig, subdomain, workDir)
+  )
+}
+
+async function deployLocked(repoFullName, branch, previewConfig, subdomain, workDir) {
+  const cloneUrl = `${config.gitea.baseUrl}/${repoFullName}.git`
   const targetDir = path.join(config.preview.deployRoot, subdomain)
 
   await cloneRepo(cloneUrl, workDir, branch, config.gitea.apiToken)
diff --git a/test/previewDeployLock.test.js b/test/previewDeployLock.test.js
new file mode 100644
index 0000000..3af586b
--- /dev/null
+++ b/test/previewDeployLock.test.js
@@ -0,0 +1,56 @@
+import { test, before, after } from 'node:test'
+import assert from 'node:assert/strict'
+import { execFileSync } from 'node:child_process'
+import fs from 'node:fs/promises'
+import os from 'node:os'
+import path from 'node:path'
+
+// 2026-09-30 (Kirschner Gebäudeservice): Projektänderung und Gitea-Webhook deployten dasselbe
+// Repo gleichzeitig, beide git clone liefen ins selbe Arbeitsverzeichnis und brachen ab.
+let tmp
+let previewDeploy
+const cwd = process.cwd()
+
+before(async () => {
+  tmp = await fs.mkdtemp(path.join(os.tmpdir(), 'preview-lock-'))
+  const repo = path.join(tmp, 'gitea', 'WEBklar', 'kunde.git')
+  const src = path.join(tmp, 'src')
+  await fs.mkdir(src, { recursive: true })
+  // Genug Inhalt, dass ein Klon Zeit braucht und sich zwei Klone überschneiden
+  for (let i = 0; i < 300; i += 1) await fs.writeFile(path.join(src, `seite-${i}.html`), `<p>${i}</p>`.repeat(200))
+  await fs.writeFile(path.join(src, 'index.html'), '<title>Kunde</title>')
+  const git = (args, dir) => execFileSync('git', args, { cwd: dir, stdio: 'ignore' })
+  git(['init', '-q', '-b', 'main'], src)
+  git(['-c', 'user.name=t', '-c', 'user.email=t@t', 'add', '.'], src)
+  git(['-c', 'user.name=t', '-c', 'user.email=t@t', 'commit', '-q', '-m', 'init'], src)
+  git(['clone', '-q', '--bare', src, repo], tmp)
+
+  await fs.mkdir(path.join(tmp, 'work'))
+  process.chdir(path.join(tmp, 'work')) // previewWorkDir() liegt unter process.cwd()
+  Object.assign(process.env, {
+    APPWRITE_ENDPOINT: 'http://127.0.0.1:9/v1',
+    APPWRITE_API_KEY: 'test-key',
+    GITEA_BASE_URL: `file://${path.join(tmp, 'gitea')}`,
+    GITEA_API_TOKEN: '',
+    PREVIEW_DEPLOY_ROOT: path.join(tmp, 'previews'),
+    TRAEFIK_DYNAMIC_DIR: '',
+  })
+  previewDeploy = await import('../server/services/previewDeploy.js')
+})
+
+after(async () => {
+  process.chdir(cwd)
+  await fs.rm(tmp, { recursive: true, force: true })
+})
+
+test('gleichzeitige Deploys desselben Repos laufen nacheinander und gelingen beide', async () => {
+  const cfg = { subdomain: 'kunde', type: 'static' }
+  const results = await Promise.allSettled([
+    previewDeploy.runDeploy('WEBklar/kunde', 'main', cfg),
+    previewDeploy.runDeploy('WEBklar/kunde', 'main', cfg),
+    previewDeploy.runDeploy('WEBklar/kunde', 'main', cfg),
+  ])
+  for (const r of results) assert.equal(r.status, 'fulfilled', r.reason?.message)
+  assert.match(await fs.readFile(path.join(tmp, 'previews', 'kunde', 'index.html'), 'utf8'), /Kunde/)
+  assert.equal((await fs.readdir(path.join(tmp, 'previews', 'kunde'))).filter((f) => f.startsWith('seite-')).length, 300)
+})