diff --git a/CLAUDE.md b/CLAUDE.md
index 3e8ebca..0db10eb 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -40,6 +40,19 @@ docker compose exec app drush riverside:rebuild
 docker compose exec app drush rrb
 ```
 
+**Note on semaphore/lock errors in logs:** During rebuilds (especially the full non-`DRUPAL_FAST` path) you may see Postgres errors like:
+
+```
+ERROR: duplicate key value violates unique constraint "semaphore____pkey"
+DETAIL: Key (name)=(state:Drupal\Core\Cache\CacheCollector) already exists.
+```
+
+This is harmless but noisy. The entrypoint and `_riverside_pt_rebuild()` proactively `TRUNCATE TABLE semaphore` at key points to suppress them. If you ever see them after a manual change, run:
+
+```bash
+docker compose exec app drush sql:query "TRUNCATE TABLE semaphore;"
+```
+
 The custom module directory is volume-mounted, so template/CSS/JS edits are live without rebuilding the Docker image. `settings.php` and `development.services.yml` are also volume-mounted.
 
 **Known gotcha:** `drush site:install` rewrites `settings.php`. Because `settings.php` is a bind-mounted file, Docker Desktop on macOS may hold a stale inode reference after the rewrite. If Drupal shows "The provided host name is not valid for this server" after a full rebuild, restart with `DRUPAL_FAST=1` to re-establish the mount without re-running site:install.
diff --git a/docker/php/entrypoint.sh b/docker/php/entrypoint.sh
index bee8f42..67609b4 100644
--- a/docker/php/entrypoint.sh
+++ b/docker/php/entrypoint.sh
@@ -37,8 +37,17 @@ else
     --account-pass="$ADMIN_PASS" \
     -y || { echo "[entrypoint] FATAL: site:install failed."; exit 1; }
   echo "[entrypoint] Drupal installed."
+
+  # Clear semaphores immediately after fresh install (prevents early
+  # duplicate key errors during first module enables + rebuild).
+  $DRUSH sql:query "TRUNCATE TABLE semaphore;" 2>/dev/null || true
 fi
 
+# Always clear stale semaphores before module enables + rebuild.
+# This is the most reliable way to avoid the duplicate key errors
+# on "semaphore" (CacheCollector, cron, state locks, etc.).
+$DRUSH sql:query "TRUNCATE TABLE semaphore;" 2>/dev/null || true
+
 echo "[entrypoint] Enabling required modules..."
 $DRUSH en -y views views_ui field_ui text options link datetime && \
   echo "[entrypoint] Core modules enabled." || echo "[entrypoint] WARNING: core modules failed."
@@ -49,6 +58,11 @@ $DRUSH en -y symfony_mailer && \
 $DRUSH en -y riverside_pt && \
   echo "[entrypoint] riverside_pt enabled." || echo "[entrypoint] WARNING: riverside_pt failed."
 
+# Clear semaphores to avoid duplicate key violations on the semaphore
+# table (e.g. during CacheCollector, cron, state operations) that can
+# occur during rapid config/entity changes in the rebuild.
+$DRUSH sql:query "TRUNCATE TABLE semaphore;" 2>/dev/null || true
+
 echo "[entrypoint] Rebuilding site structure from code (riverside:rebuild)..."
 $DRUSH riverside:rebuild || echo "[entrypoint] WARNING: riverside:rebuild encountered an issue."
 
@@ -63,6 +77,10 @@ $DRUSH config:set system.site page.front /home -y && \
 
 npm run build --prefix /var/www/html >/dev/null 2>&1 && echo "[entrypoint] Tailwind built." || echo "[entrypoint] WARNING: Tailwind build failed."
 
+# One more semaphore clear before the final cache rebuild (common source
+# of the duplicate key errors seen in logs).
+$DRUSH sql:query "TRUNCATE TABLE semaphore;" 2>/dev/null || true
+
 $DRUSH cache:rebuild >/dev/null 2>&1 && echo "[entrypoint] Cache rebuilt."
 
 if [ "${DEBUG:-false}" = "true" ]; then
diff --git a/web/modules/custom/riverside_pt/riverside_pt.install b/web/modules/custom/riverside_pt/riverside_pt.install
index 46b749d..f6895ca 100644
--- a/web/modules/custom/riverside_pt/riverside_pt.install
+++ b/web/modules/custom/riverside_pt/riverside_pt.install
@@ -26,6 +26,18 @@ function riverside_pt_install() {
  * and navigation used by the Docker entrypoint.
  */
 function _riverside_pt_rebuild(): void {
+  // Clear any stale semaphore locks. This prevents duplicate key violations
+  // on the "semaphore" table (e.g. "state:Drupal\Core\Cache\CacheCollector",
+  // "cron") during rebuilds. These occur because rapid entity/config
+  // changes trigger cache collectors and lock acquisitions concurrently.
+  // Safe to run even on initial install (table may not exist yet).
+  try {
+    \Drupal::database()->truncate('semaphore')->execute();
+  }
+  catch (\Exception $e) {
+    // Ignore if table doesn't exist yet.
+  }
+
   $entity_type_manager = \Drupal::entityTypeManager();
   $storage_node_type = $entity_type_manager->getStorage('node_type');
   $storage_role = $entity_type_manager->getStorage('user_role');