Index: management/univention-management-console-module-udm/umc/python/udm/__init__.py
===================================================================
--- management/univention-management-console-module-udm/umc/python/udm/__init__.py	(Revision 73141)
+++ management/univention-management-console-module-udm/umc/python/udm/__init__.py	(Arbeitskopie)
@@ -368,6 +368,8 @@
 					result.append({'$dn$': dn, 'success': True})
 				except UDM_Error as e:
 					result.append({'$dn$': e.dn, 'success': False, 'details': str(e)})
+				except udm_errors.valueError as e:
+					result.append({'$dn$': None, 'success': False, 'details': str(e)})
 
 			return result
 
Index: management/univention-management-console-module-udm/umc/python/udm/udm_ldap.py
===================================================================
--- management/univention-management-console-module-udm/umc/python/udm/udm_ldap.py	(Revision 73141)
+++ management/univention-management-console-module-udm/umc/python/udm/udm_ldap.py	(Arbeitskopie)
@@ -37,6 +37,7 @@
 import threading
 import gc
 import functools
+import pwd
 
 from univention.management.console import Translation
 from univention.management.console.protocol.definitions import BAD_REQUEST_UNAUTH
@@ -334,6 +335,11 @@
 	@LDAP_Connection
 	def create(self, ldap_object, container=None, superordinate=None, ldap_connection=None, ldap_position=None):
 		"""Creates a LDAP object"""
+		username = ldap_object['username']
+		posix_users = [user.pw_name for user in pwd.getpwall()]
+		if username in posix_users:
+			raise udm_errors.valueError(_('The username "%s" is already assigned to an existing POSIX user.') % username)
+
 		if superordinate not in (None, 'None'):
 			try:
 				ldap_position.setDn(superordinate)