CORS support adds CORS-related response headers twice

[tjquinno]

Environment Details

• Helidon Version: 2.x
• Helidon SE
• JDK version: n/a
• OS: n/a
• Docker version (if applicable): n/a

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The CORS support adds headers to the response as needed, but it currently does so twice for the same headers. This causes problems if you try the reproducer described in #1562; you get an error about Access-Control-Allow-Origin having multiple values.