Retrurn error if exist duplicate key in filter option

[MIBc]

Fixes #569

Proposed Changes

*Return error if user set duplicate key in --filter option. For example, kn trigger create --filter type=bla --filter type=blub test.

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[navidshaikh]

/ok-to-test

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-client-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/kn/commands/trigger/update_flags.go	96.2%	96.7%	0.5

[MIBc]

/retest

[navidshaikh]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: MIBc, navidshaikh

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[rhuss]

thanks, looks good to me !

While we are at it: both methods you changed have quite some overlap. I would prefer to have a single method, which takes another boolean parameter, whether to check for filters to delete. Thinking again, we could even keep the removal of types even for create as a no-op, as you can't easily use filter key with a trailing dash anyway (how would you update those ?). So we could keep a single method for both create & update.

Would you mind to combine these two methods already within this PR ?

[rhuss]

or then maybe in a next PR ;-)

[rhuss]

(was too slow ...)

[MIBc]

Sure, I will do.

[rhuss]

@MIBc If you like, you could also jump on #577 which I just have created. I think this would be the most comprehensive solution.

[MIBc]

Yes. I will try. Thanks!