diff --git a/setup.cfg b/setup.cfg
index 10472d502a..88fee9d4fc 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -36,7 +36,6 @@ include_trailing_comma = true
 # see mypy.readthedocs.io/en/stable/command_line.html
 implicit_optional = True
 ignore_missing_imports = True
-implicit_optional = True
 files = esmvalcore, tests
 
 [yapf]