Deduplicate SBOMs in static reachability output

What does this MR do and why?

We currently create duplicate outputs when running Static Reachability Analysis on the same SBOM with multiple enrichers. See Static Reachability multiple languages cause be... (gitlab-org/gitlab#557556 - closed).

This MR checks whether an SBOM object already exists in the enrichers output list before adding it. Since SBOMs are passed by reference, an SBOM that was already added to the list may later be modified again without having to re-add it.

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