bug: duplicate initialization drops existing listeners

[sirugh]

Description

Rather than try to gracefully merge an existing acdl (post-augment) with one loaded later, I simply check for an existing property and, if it exists, skip initialization entirely.

Related Issue

Closes #126

Motivation and Context

See #126

How Has This Been Tested?

Using the sandbox from the issue, I proxied the unpkg import to a local built file that included the changes in this PR.

Doing so logged the warning and allowed the second event to be observed by the original listener without having to re-attach it.

See screenshot below.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[sirugh]

I did not update docs/tests as I'd like to get feedback first.

[sirugh]

@jckautzmann are you the maintainer for this? If not, who can I ping for a review/merge :)

[jckautzmann]

@sirugh Thx for your contribution!