diff --git a/numberify/numberify.py b/numberify/numberify.py
index 0cff9bc..fbda4ba 100644
--- a/numberify/numberify.py
+++ b/numberify/numberify.py
@@ -31,7 +31,11 @@ def numberify_file(self, filename):
             fd = open(filename, 'r+')
             lines = fd.readlines()
             for i in range(len(lines)):
-                lines[i] = str(i+1) + ' ' + lines[i]
+                if lines[i][0:len(str(i+1))] != str(i+1):
+                    lines[i] = str(i+1) + ' ' + lines[i]
+                else:
+                    print "File already numbered"
+                    return False
             fd.seek(0)
             fd.writelines(lines)
             fd.close()
diff --git a/test/test_numberify.py b/test/test_numberify.py
index dd95dac..512d4bb 100644
--- a/test/test_numberify.py
+++ b/test/test_numberify.py
@@ -12,7 +12,10 @@ def test_numberify_file():
     '''testing numberify_file'''
     num = Numberify()
     copyfile(file1, file2)
-    num.numberify_file(file2)
+    result = num.numberify_file(file2)
+    assert result is True
+    result = num.numberify_file(file2)
+    assert result is False
     fd = open(file2, 'r')
     lines = fd.readlines()
     length = len(lines)