duplicate output messages

[hjoliver]

While testing #6046 I noticed we can assign the same output message to different triggers:

[scheduling]
  [[graph]]
      R1 = "foo:x? & foo:y? => bar"
[runtime]
  [[bar]]
  [[foo]]
     script = "cylc message 'shake n bake'"
     [[[outputs]]]
          x = "shake n bake"
          y = "shake n bake"

This ⏫ does the right thing and completes both outputs, but I'm pretty sure it would be a mistake if found in the wild.

You might want to add the fix into #6046 @oliver-sanders ?