diff --git a/pytoml/parser.py b/pytoml/parser.py
index 189589e..3cbd177 100644
--- a/pytoml/parser.py
+++ b/pytoml/parser.py
@@ -370,7 +370,7 @@
             toks.expect('=', 'expected_equals')
             type, v = read_value()
             if k in scope:
-                toks.error('duplicate_keys')
+                toks.error('duplicate_keys. Key "{}" was used more than once.'.format(k))
             scope[k] = v
             toks.expect('\n', 'expected_eol')
         elif toks.consume('\n'):