diff --git a/.github/scripts/test-unique-hostname-precheck.sh b/.github/scripts/test-unique-hostname-precheck.sh
new file mode 100755
index 0000000..a2c5f7c
--- /dev/null
+++ b/.github/scripts/test-unique-hostname-precheck.sh
@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+set -Eeuo pipefail
+
+repo_root="$(git rev-parse --show-toplevel)"
+site_play="$repo_root/site.yml"
+
+# #636: verify the "Pre tasks" play asserts that all k3s_cluster hosts have
+# unique hostnames, so a duplicate-hostname inventory fails fast instead of
+# silently breaking node registration/joining.
+grep -Fq -- 'Verify all cluster nodes have unique hostnames' "$site_play" || {
+  printf 'site.yml is missing the unique-hostname preflight check\n' >&2
+  exit 1
+}
+
+# The check must deduplicate the cluster hostname list via the `unique` filter
+# and compare lengths, i.e. groups['k3s_cluster'] must be referenced.
+grep -Fq -- "groups['k3s_cluster']" "$site_play" || {
+  printf 'unique-hostname check does not iterate the k3s_cluster group\n' >&2
+  exit 1
+}
+
+if ! grep -Eq -- 'cluster_hostnames.*\|.*unique|\| unique' "$site_play"; then
+  printf 'unique-hostname check does not deduplicate the hostname list\n' >&2
+  exit 1
+fi
+
+printf 'Unique hostname preflight regression test passed\n'
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 08ab344..53e78b6 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -62,6 +62,12 @@ repos:
         language: system
         pass_filenames: false
         files: ^roles/k3s_server/tasks/(main|join_master)\.yml$|^\.github/scripts/test-k3s-server-bootstrap\.sh$
+      - id: unique-hostname-precheck-test
+        name: Unique hostname precheck test
+        entry: .github/scripts/test-unique-hostname-precheck.sh
+        language: system
+        pass_filenames: false
+        files: ^site\.yml$|^\.github/scripts/test-unique-hostname-precheck\.sh$
       - id: cilium-bgp-manifest-test
         name: Cilium BGP manifest test
         entry: python3 .github/scripts/test-cilium-bgp-manifest.py
diff --git a/README.md b/README.md
index d50f8e9..4dbc855 100644
--- a/README.md
+++ b/README.md
@@ -50,6 +50,10 @@ Supported processor architectures are:
 - Server and agent nodes should support passwordless SSH access. Otherwise, pass `--ask-pass --ask-become-pass` to
   each playbook command.
 
+- Every node in the cluster must have a **unique hostname**. k3s registers each node keyed by its hostname, so
+  two nodes with the same hostname cannot join the cluster. `site.yml` asserts this up front and fails fast if
+  any duplicate is found.
+
 ## 🚀 Getting Started
 
 ### 🍴 Preparation
diff --git a/site.yml b/site.yml
index 1fe2274..8a11ce7 100644
--- a/site.yml
+++ b/site.yml
@@ -8,6 +8,23 @@
         msg: >
           "Ansible is out of date. See here for more info: https://docs.technotim.com/posts/ansible-automation/"
 
+    - name: Verify all cluster nodes have unique hostnames
+      ansible.builtin.assert:
+        that: (cluster_hostnames | unique | length) == (cluster_hostnames | length)
+        msg: >-
+          k3s nodes must have unique hostnames. Found a duplicate in:
+          {{ cluster_hostnames | unique }}. Each node registers in the cluster
+          keyed by its hostname, so matching hostnames prevent nodes from joining.
+      vars:
+        cluster_hostnames: >-
+          {{
+            groups['k3s_cluster']
+            | map('extract', hostvars, 'ansible_hostname')
+            | list
+          }}
+      run_once: true
+      when: "'k3s_cluster' in groups"
+
 - name: Prepare Proxmox cluster
   hosts: proxmox
   gather_facts: true