6-9 - There are duplicate serial number IDs

Possible Reason

1. An empty object is returned.
2. The custom serial number class, org.apache.dubbo.common.serialize.Serialization#getContentTypeId is duplicated with the built-in system, At this time, when loading, the first loaded SPI instance shall prevail. Other items will be skipped.

Troubleshooting and resolution steps

1. Check the return result.
2. The built-in value can refer to the class org.apache.dubbo.common.serialize.Constants