Callback called twice

[felipe-najson-ntf]

Solves the issue that callback was calling twice.

• All tests working.
• Related to issue Events are not sent in a Vercel serverless function #303 Issue using analytics within Lambda functions #305 and enque callback called twice when flush is triggered #308.
• An improvement from pr Flush was making callback to be called twice #299

[benjaminhoffman]

@felipe-najson-ntf when will this change be deployed to analytics-node? i am using 6.0.0 and callbacks.forEach(callback => callback(err)) is causing me issues... thank you! 🙏