Fix duplicate export modifier diagnostic message

- Changed diagnostic from "export modifier must precede declare modifier" to "export modifier already seen"
- Skip adding duplicate export modifier to the modifiers list to avoid secondary validation error
- Add test case for duplicate export modifier
- Handle empty modifiers case when all modifiers are skipped

Co-authored-by: camc314 <[email protected]>

Author: Copilot

crates/oxc_parser/src/modifiers.rs

@@ -339,7 +339,11 @@ impl<'a> ParserImpl<'a> {
             flags.set(modifier_flags, true);
             modifiers.push(modifier);
         }
-        Modifiers::new(Some(modifiers), flags)
+        if modifiers.is_empty() {
+            Modifiers::empty()
+        } else {
+            Modifiers::new(Some(modifiers), flags)
+        }
     }
 
     fn at_modifier(&mut self) -> bool {

tasks/coverage/misc/fail/duplicate-export-modifier.ts

@@ -0,0 +1,2 @@
+export export function foo() {
+}