diff --git a/pip_run/scripts.py b/pip_run/scripts.py
index 95f1e36..b42f584 100644
--- a/pip_run/scripts.py
+++ b/pip_run/scripts.py
@@ -162,7 +162,8 @@ def _read(self, var_name):
         """
         Read a variable from self.script by parsing the AST.
 
-        Raises ValueError if the variable is not found.
+        Raises ValueError if the variable is not found or if it
+        appears more than once.
         """
         mod = ast.parse(self.script)
         (node,) = (
diff --git a/tests/test_scripts.py b/tests/test_scripts.py
index 877c0a5..0326b4c 100644
--- a/tests/test_scripts.py
+++ b/tests/test_scripts.py
@@ -56,6 +56,15 @@ def test_reads_files_with_attribute_assignment(self):
         )
         assert scripts.DepsReader(script).read() == ['foo']
 
+    def test_duplicate_definition(self):
+        script = textwrap.dedent(
+            """
+            __requires__=['foo']
+            __requires__=['bar']
+            """
+        )
+        assert scripts.DepsReader(script).read() == []
+
     def test_reads_files_with_multiple_assignment(self):
         script = textwrap.dedent(
             """