Fix PipelineTask double-inserting RTVIProcessor when custom RTVIObserver is provided

When the user places an RTVIProcessor inside their pipeline and provides
a custom RTVIObserver subclass in observers, PipelineTask correctly
detects both and logs "skipping default ones." However it then
unconditionally prepends self._rtvi to the pipeline, causing the
processor to appear twice in the frame chain.

Track whether the RTVIProcessor was found externally (inside the user
pipeline) vs created internally. Only prepend it when created internally.

Fixes #3867

changelog/3867.fixed.md

@@ -0,0 +1 @@
+- Fixed `PipelineTask` double-inserting `RTVIProcessor` into the frame chain when the user provides both an `RTVIProcessor` in the pipeline and a custom `RTVIObserver` subclass in observers.

src/pipecat/pipeline/task.py

@@ -330,6 +330,7 @@ class PipelineTask(BasePipelineTask):
 
         # RTVI support
         self._rtvi = None
+        self._rtvi_external = False
         external_rtvi = self._find_processor(pipeline, RTVIProcessor)
         external_observer_found = any(isinstance(o, RTVIObserver) for o in observers)
 
@@ -349,6 +350,7 @@ class PipelineTask(BasePipelineTask):
                 "They are both added by default, no need to add them yourself."
             )
             self._rtvi = external_rtvi
+            self._rtvi_external = True
         elif enable_rtvi:
             self._rtvi = rtvi_processor or RTVIProcessor()
             observers.append(self._rtvi.create_rtvi_observer(params=rtvi_observer_params))
@@ -388,7 +390,13 @@ class PipelineTask(BasePipelineTask):
         # allows us to receive and react to downstream frames.
         source = PipelineSource(self._source_push_frame, name=f"{self}::Source")
         sink = PipelineSink(self._sink_push_frame, name=f"{self}::Sink")
-        processors = [self._rtvi, pipeline] if self._rtvi else [pipeline]
+        # Only prepend the RTVIProcessor if we created it ourselves. When the
+        # user already placed it inside their pipeline we must not insert it
+        # again or it will appear twice in the frame chain.
+        if self._rtvi and not self._rtvi_external:
+            processors = [self._rtvi, pipeline]
+        else:
+            processors = [pipeline]
         self._pipeline = Pipeline(processors, source=source, sink=sink)
 
         # The task observer acts as a proxy to the provided observers. This way,