diff --git a/llm/templates.py b/llm/templates.py
index 1919e0484..7d0c162f4 100644
--- a/llm/templates.py
+++ b/llm/templates.py
@@ -76,7 +76,7 @@ def interpolate(cls, text: str | None, params: dict[str, Any]) -> str | None:
         # Confirm all variables in text are provided
         string_template = string.Template(text)
         vars = cls.extract_vars(string_template)
-        missing = [p for p in vars if p not in params]
+        missing = list(dict.fromkeys(p for p in vars if p not in params))
         if missing:
             raise cls.MissingVariables(
                 "Missing variables: {}".format(", ".join(missing))
diff --git a/tests/test_templates.py b/tests/test_templates.py
index 9ef2d8888..b9cef9db0 100644
--- a/tests/test_templates.py
+++ b/tests/test_templates.py
@@ -21,6 +21,9 @@
         ("S: $input", "system", None, {}, "S: input", "system", None),
         ("No vars", None, None, {}, "No vars", None, None),
         ("$one and $two", None, None, {}, None, None, "Missing variables: one, two"),
+        # A variable reused in the template is reported once, not once per use.
+        ("$name and $name", None, None, {}, None, None, "Missing variables: name"),
+        ("$a $b $a", None, None, {}, None, None, "Missing variables: a, b"),
         ("$one and $two", None, None, {"one": 1, "two": 2}, "1 and 2", None, None),
         ("$one and $two", None, {"one": 1}, {"two": 2}, "1 and 2", None, None),
         ("$one and $$2", None, None, {"one": 1}, "1 and $2", None, None),