[BugFix] Prevent exporting duplicate OpenTelemetry spans (#9017)

vllm/engine/llm_engine.py

@@ -1245,7 +1245,7 @@ class LLMEngine:
                               skip)
 
             # Tracing
-            self.do_tracing(scheduler_outputs)
+            self.do_tracing(scheduler_outputs, finished_before)
 
         return None
 
@@ -1840,11 +1840,18 @@ class LLMEngine:
     def is_tracing_enabled(self) -> bool:
         return self.tracer is not None
 
-    def do_tracing(self, scheduler_outputs: SchedulerOutputs) -> None:
+    def do_tracing(self,
+                   scheduler_outputs: SchedulerOutputs,
+                   finished_before: Optional[List[int]] = None) -> None:
         if self.tracer is None:
             return
 
-        for scheduled_seq_group in scheduler_outputs.scheduled_seq_groups:
+        for idx, scheduled_seq_group in enumerate(
+                scheduler_outputs.scheduled_seq_groups):
+            # Skip double tracing when using async output proc
+            if finished_before and idx in finished_before:
+                continue
+
             seq_group = scheduled_seq_group.seq_group
             if seq_group.is_finished():
                 self.create_trace_span(seq_group)