fix duplicate syntax error

Author: ntBre

crates/ruff/tests/lint.rs

@@ -2779,8 +2779,7 @@ l[(x:=1):-1]
     test.py:6:5: SyntaxError: Cannot use unparenthesized assignment expression on Python 3.9 (syntax was added in Python 3.10)
     test.py:7:5: SyntaxError: Cannot use unparenthesized assignment expression on Python 3.9 (syntax was added in Python 3.10)
     test.py:10:3: SyntaxError: Unparenthesized named expression cannot be used here
-    test.py:10:3: SyntaxError: Cannot use unparenthesized assignment expression on Python 3.9 (syntax was added in Python 3.10)
-    Found 6 errors.
+    Found 5 errors.
 
     ----- stderr -----
     "

crates/ruff_python_parser/src/parser/expression.rs

@@ -874,16 +874,16 @@ impl<'src> Parser<'src> {
             let lower =
                 self.parse_named_expression_or_higher(ExpressionContext::starred_conditional());
 
-            if self.options.target_version < PythonVersion::PY310
-                && lower.is_unparenthesized_named_expr()
-            {
-                self.add_unsupported_syntax_error(
-                    UnsupportedSyntaxErrorKind::UnparWalrusBefore310,
-                    lower.range(),
-                );
-            }
-
             if self.at_ts(NEWLINE_EOF_SET.union([TokenKind::Rsqb, TokenKind::Comma].into())) {
+                // check this here because if we don't return, we will emit a ParseError instead
+                if self.options.target_version < PythonVersion::PY310
+                    && lower.is_unparenthesized_named_expr()
+                {
+                    self.add_unsupported_syntax_error(
+                        UnsupportedSyntaxErrorKind::UnparWalrus,
+                        lower.range(),
+                    );
+                }
                 return lower.expr;
             }