src/jdk.scripting.nashorn/share/classes/jdk/nashorn/internal/codegen/AssignSymbols.java

@@ -341,11 +341,11 @@
                 if (!isLocal(function, symbol)) {
                     // Not defined in this function. Create a new definition.
                     symbol = null;
                 } else if (symbol.isParam()) {
                     // Duplicate parameter. Null return will force an error.
-                    throw new AssertionError("duplicate parameter");
+                    throwParserException(ECMAErrors.getMessage("syntax.error.duplicate.parameter", name), origin);
                 }
             } else if (isVar) {
                 if (isBlockScope) {
                     // Check redeclaration in same block
                     if (symbol.hasBeenDeclared()) {