Detect duplicated parameters in function definitions.

tests/call_fn.rs

@@ -1,5 +1,22 @@
 #![cfg(not(feature = "no_function"))]
-use rhai::{Engine, EvalAltResult, INT};
+use rhai::{Engine, EvalAltResult, ParseError, ParseErrorType, INT};
+
+#[test]
+fn test_fn() -> Result<(), EvalAltResult> {
+    let mut engine = Engine::new();
+
+    // Expect duplicated parameters error
+    match engine
+        .compile("fn hello(x, x) { x }")
+        .expect_err("should be error")
+        .error_type()
+    {
+        ParseErrorType::FnDuplicateParam(f, p) if f == "hello" && p == "x" => (),
+        _ => assert!(false, "wrong error"),
+    }
+
+    Ok(())
+}
 
 #[test]
 fn test_call_fn() -> Result<(), EvalAltResult> {