Fix duplicate task triggers

[MetRonnie]

Closes #6619 (comment)

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• No dependency changes
• Tests are included
• Changelog entry included if this is a change that can affect users
• No docs needed
• Opened on master as this is not an urgent fix

[MetRonnie]

(Can change to 8.4.x if desired)

[oliver-sanders]

(Can change to 8.4.x if desired)

Happy to leave here, probs more appropriate for a minor release.

Profiling results show a reasonable reduction in CPU 🎉.

method

#!Jinja2

[scheduler]
    allow implicit tasks = True

[task parameters]
    run = 1..{{ N }}

[scheduling]
    initial cycle point = 2000
    final cycle point = 2000-01-03
    runahead limit = P0
    [[queues]]
        [[[default]]]
            limit = 500
    [[graph]]
        P1D = """
            @wall_clock => start => foo<run> => bar
            bar:run | foo<run> => baz
        """

[runtime]
    [[root]]
        script = true
        run mode = skip
        [[[skip]]]
            # default run length = PT0S
            outputs = succeeded, run
        [[[outputs]]]
            run = run

$ /usr/bin/time -v cylc vip --no-detach --mode=live -s 'N=<number>'

[wxtim]

Feel free to merge, but consider my suggestion that you test repr?

[hjoliver]

Test added, merging...