diff --git a/ldapcherry/__init__.py b/ldapcherry/__init__.py
index 6e81d7d..7b0acea 100644
--- a/ldapcherry/__init__.py
+++ b/ldapcherry/__init__.py
@@ -168,7 +168,7 @@ class ldapCherry(object):
         else:
             return logging.INFO
 
-     def _reraise(self, exception):
+    def _reraise(self, exception):
         """ reraise a given exception"""
         raise exception
     
diff --git a/ldapcherry/exceptions.py b/ldapcherry/exceptions.py
index f8e91cf..21a0876 100644
--- a/ldapcherry/exceptions.py
+++ b/ldapcherry/exceptions.py
@@ -9,3 +9,16 @@ class MissingParameter(Exception):
     def __init__(self, section, key):
         self.section = section
         self.key = key
+
+class MissingKey(Exception):
+    def __init__(self, key):
+        self.key = key
+
+class DumplicateRoleKey(Exception):
+    def __init__(self, role):
+        self.role = role
+
+class DumplicateRoleContent(Exception):
+    def __init__(self, role1, role2):
+        self.role1 = role1
+        self.role2 = role2
diff --git a/ldapcherry/pyyamlwrapper.py b/ldapcherry/pyyamlwrapper.py
new file mode 100644
index 0000000..e0af1c9
--- /dev/null
+++ b/ldapcherry/pyyamlwrapper.py
@@ -0,0 +1,72 @@
+# -*- coding: utf-8 -*-
+
+import os
+import sys
+from yaml.error import *
+from yaml.nodes import *
+from yaml.reader import *
+from yaml.scanner import *
+from yaml.parser import *
+from yaml.composer import *
+from yaml.constructor import *
+from yaml.resolver import *
+
+
+class RelationError(Exception):
+    def __init__(self, key, value):
+        self.key   = key
+        self.value = value
+
+class DumplicatedKey(Exception):
+    def __init__(self, host, key):
+        self.host   = host
+        self.key    = key
+
+import yaml
+
+try:
+        from yaml import CLoader as Loader, CDumper as Dumper
+except ImportError:
+        from yaml import Loader, Dumper
+
+# PyYaml wrapper that loads yaml files throwing an exception 
+#if a key is dumplicated
+class MyLoader(Reader, Scanner, Parser, Composer, Constructor, Resolver):
+
+    def __init__(self, stream):
+        Reader.__init__(self, stream)
+        Scanner.__init__(self)
+        Parser.__init__(self)
+        Composer.__init__(self)
+        Constructor.__init__(self)
+        Resolver.__init__(self)
+
+    def construct_mapping(self, node, deep=False):
+        exc = sys.exc_info()[1]
+        if not isinstance(node, MappingNode):
+            raise ConstructorError(None, None,
+                    "expected a mapping node, but found %s" % node.id,
+                    node.start_mark)
+        mapping = {}
+        for key_node, value_node in node.value:
+            key = self.construct_object(key_node, deep=deep)
+            try:
+                hash(key)
+            except TypeError:
+                raise ConstructorError("while constructing a mapping", node.start_mark,
+                        "found unacceptable key (%s)" % exc, key_node.start_mark)
+            value = self.construct_object(value_node, deep=deep)
+            if key in mapping:
+                raise DumplicatedKey(key, '')
+            mapping[key] = value
+        return mapping
+
+
+def loadNoDump(stream):
+    loader = MyLoader(stream)
+    try:
+        return loader.get_single_data()
+    finally:
+        loader.dispose()
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4
diff --git a/ldapcherry/roles.py b/ldapcherry/roles.py
index ccefbf5..f4627c8 100644
--- a/ldapcherry/roles.py
+++ b/ldapcherry/roles.py
@@ -8,19 +8,20 @@
 import os
 import sys
 
-try:
-    from yaml import CLoader as Loader, CDumper as Dumper
-except ImportError:
-    from yaml import Loader, Dumper
+from ldapcherry.pyyamlwrapper import loadNoDump
+
 
 class Roles:
 
     def __init__(self, role_file):
-        pass
+        stream = open(role_file, 'r')
+        self.roles_raw = loadNoDump(stream)
+        stream.close()
+        self._nest()
 
-    def _nest(self, role_file):
+    def _nest(self):
         """nests the roles (creates roles hierarchy)"""
-        pass
+        self.roles = self.roles_raw
 
     def write(self, out_file):
         """write the nested role hierarchy to a file"""