When the CDC Capture job runs it fails with ‘Cannot insert duplicate key row in object cdc.Table_CT’ with unique index ‘CT Clustered Index’.
This index is a sql server cdc created index (columns __start_lsn, __$command_id, __$seqval, __$operation)

Any ideas how to simulate this to trap why this happens, or a way to overcome these issues with cdc? Do I need to enable traceflags, or CU installed, etc?

The only solution to get passed this error was to disable cdc on the database and re-enable the database and then the affected table. In this scenario, we were fortunate to do this, but this can not be done on other environments