diff --git a/README.md b/README.md
index ef8a8fa..0d3073a 100644
--- a/README.md
+++ b/README.md
@@ -23,6 +23,7 @@ Below are the concrete rules governing sections, keys, values, and supported dat
 - All (sub-) sections may only be defined once.
 - (Sub-) sections can be empty.
 - Keys within (sub-) sections use `key = value` syntax and are seperated by new-lines.
+- Keys within (sub-) sections may only be defined once.
 - Key and (sub-) section names may only contain `a-z`, `A-Z`, `0-9`, and `_`.
 - Values may never reach over multiple lines.
 - Empty keys (i.e., keys without values) are not allowed.