Workflow runs a node twice when a list of routes matches two edges to it #8072

🔴 Required Information

Describe the Bug:
Since 460715b, a workflow graph may have several edges between the same two nodes as long as their routes are disjoint, for example a routing map (triage, {"billing": handle, "refund": handle}). When a node emits a list of routes that matches more than one of those edges, Graph.get_next_pending_nodes returns the target once per matched edge, so the target runs once per edge:

• A plain node runs twice (handle@1 and handle@2, each in its own sub-branch, because _buffer_downstream_triggers treats the two entries as a fan-out).
• A JoinNode whose only predecessor is that node fires twice, and so does everything after it.

A single edge with route=["billing", "refund"] to the same node runs it once.

Steps to Reproduce:

1. Install google-adk.
2. Run the code below.

Expected Behavior:
Each target runs once.

-- fan_in
handle ran with ticket
-- join
notify ran

Observed Behavior:

-- fan_in
handle ran with ticket
handle ran with ticket
-- join
notify ran
notify ran

Environment Details:

• ADK Library Version (pip show google-adk): main at 1973796 (2.14.0)
• Desktop OS: macOS
• Python Version (python -V): 3.14.7

Model Information:

• Are you using LiteLLM: No
• Which model is being used: N/A (no model call is needed to reproduce)

🟡 Optional Information

Regression:
Yes. Before 460715b, graph validation rejected a second edge between the same two nodes.

Minimal Reproduction Code:

import asyncio

from google.adk.events.event import Event
from google.adk.runners import InMemoryRunner
from google.adk.workflow import JoinNode
from google.adk.workflow import START
from google.adk.workflow import Workflow
from google.genai import types


def triage():
  return Event(output="ticket", route=["billing", "refund"])


def handle(node_input: str):
  print("handle ran with", node_input)


def notify():
  print("notify ran")


async def run(name, edges):
  runner = InMemoryRunner(app_name=name, agent=Workflow(name=name, edges=edges))
  session = await runner.session_service.create_session(
      app_name=name, user_id="u"
  )
  print(f"-- {name}")
  async for _ in runner.run_async(
      user_id="u",
      session_id=session.id,
      new_message=types.Content(role="user", parts=[types.Part(text="hi")]),
  ):
    pass


async def main():
  await run(
      "fan_in", [(START, triage), (triage, {"billing": handle, "refund": handle})]
  )
  gate = JoinNode(name="gate")
  await run(
      "join",
      [
          (START, triage),
          (triage, {"billing": gate, "refund": gate}),
          (gate, notify),
      ],
  )


asyncio.run(main())

How often has this issue occurred?:

• Always (100%)