diff --git a/itstool.in b/itstool.in
index d0510a9..7715dfc 100755
--- a/itstool.in
+++ b/itstool.in
@@ -194,13 +194,14 @@ class Message (object):
     def add_source (self, source):
         if not isinstance(source, unicode):
             source = unicode(source, 'utf-8')
-        self._sources.append(source)
+        if source not in self._sources:
+            self._sources.append(source)
 
     def get_sources (self):
         return self._sources
 
     def add_comment (self, comment):
-        if comment is not None:
+        if comment is not None and comment not in self._comments:
             self._comments.append(comment)
 
     def get_comments (self):