I haven't been able to duplicate this issue with the JDBC Store node connecting to Snowflake. In general, the "IllegalArgumentException" error with a reason "Field X already exists in metadata" means that the field specified was already in the node's output pin and couldn't be added again. This error stems from the node itself rather than Snowflake. Usually this is seen with nodes like the Create Data note with duplicate columns.
The only output that the JDBC Store makes by default is a list of the records that errored during the database insert. There's an optional LoadedData property too, which also lists the successful records. I've tried both of these output pins and wasn't able to get that error message.
I created a support ticket so that we can get logs to look at this deeper. Our team will be in contact with you on that ticket shortly.