diff --git a/src/tools.py b/src/tools.py
index 740f92c9d..488ed516b 100644
--- a/src/tools.py
+++ b/src/tools.py
@@ -16,6 +16,7 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program. If not, see <http://www.gnu.org/licenses/>.
 #
+import pwd
 import re
 import os
 import subprocess
@@ -174,6 +175,12 @@ def tools_postinstall(
             raw_msg=True,
         )
 
+    # Crash early if the username is already a system user, which is
+    # a common confusion. We don't want to crash later and end up in an half-configured state.
+    all_existing_usernames = {x.pw_name for x in pwd.getpwall()}
+    if username in all_existing_usernames:
+        raise YunohostValidationError("system_username_exists")
+
     if username in ADMIN_ALIASES:
         raise YunohostValidationError(
             f"Unfortunately, {username} cannot be used as a username", raw_msg=True