Add checking for duplicate declared resources

[GregDritschler]

Fixes #2241

Changes

Adds checking for duplicated resource names in a pipeline's declared resources.

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[tekton-robot]

Hi @GregDritschler. Thanks for your PR.

I'm waiting for a tektoncd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[vdemeester]

/ok-to-test

[vdemeester]

/lgtm

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1alpha1/pipeline_validation.go	96.2%	96.3%	0.1
pkg/apis/pipeline/v1beta1/pipeline_validation.go	93.5%	93.8%	0.3

[tekton-robot]

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