diff --git a/src/files.py b/src/files.py
index af0702c..1e851c4 100644
--- a/src/files.py
+++ b/src/files.py
@@ -37,7 +37,7 @@ class PireneaFiles(object):
         for path, _dirs, files in os.walk(self.folder):
             for filename in files:
                 if filename.startswith("P"):
-                    print("File %s has already a prefix.")
+                    print("File {0} has already a prefix.".format(filename))
                 else:
                     new_filename = prefix + filename
                     os.rename(os.path.join(path, filename), os.path.join(path, new_filename))
diff --git a/tests/test_files.py b/tests/test_files.py
index b352fe1..1f261a1 100644
--- a/tests/test_files.py
+++ b/tests/test_files.py
@@ -40,7 +40,12 @@ class PireneaFilesTestCase(unittest.TestCase):
         with tempfile.TemporaryDirectory(prefix="PIRENEA_DATA") as tmpdirname:
             with tempfile.TemporaryFile(dir=tmpdirname) as tf:
                 self.pirenea = PireneaFiles(tmpdirname)
+                # Check add prefix
                 self.pirenea.add_prefix("P0")
+                # Check if add prefix has no effect if already added
+                self.pirenea.add_prefix("P0")
+                self.pirenea.remove_prefix("P0")
+                # Check if remove prefix has no effect if file has no prefix
                 self.pirenea.remove_prefix("P0")
                 """Check if temporary file tf is unchanged"""
                 self.assertTrue(os.path.isfile(tf.name))
--
libgit2 0.21.2