diff --git a/spec/2025-09.md b/spec/2025-09.md
index ca1f2a2b..293e3c90 100644
--- a/spec/2025-09.md
+++ b/spec/2025-09.md
@@ -762,6 +762,8 @@ Test cases and [test data groups](#test-data-groups) will be used in lexicograph
 It is good practice to use a numbered prefix such as `00`, `01`, `02`, `03`, and so on, to get the desired order of test cases, while keeping the file names descriptive.
 Remember that the numbered prefixes should be zero padded to the same length to get the expected lexicographical order.
 
+Test case file names (the base name of the `.in` file) must be unique across the entire `data/` directory, unless the test cases are equivalent.
+
 ### Test data groups
 
 The secret data may be subdivided into test data groups.