fix: Connections can be highlighted multiple times

[koenvanwijk]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide
• I ran npm run format and npm run lint

The details

Resolves

fixes #6501

Proposed Changes

In connection highlight, if highlightPath already exists, then silently return early because it's already highlighted.

Behavior Before Change

Calling highlight twice will leave the connection highlight.

Behavior After Change

The highlight is not applied twice, so can be removed.

Reason for Changes

Test Coverage

Manually tested, see pictures.

Documentation

No documentation update is needed, now it works as expected and documented.

Additional Information

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[rachel-fenichel]

Thanks!