What are “is already registered” WARN messages when launching spark-shell?

Question

I have installed spark/hadoop under windows. When I execute spark-shell, I get these "already registered" warnings.

I am not sure if it is something I need to worry about

C:\Users\Administrator\spark-2.2.0-bin-hadoop2.7\bin>spark-shell
Using Spark's default log4j profile: org/apache/spark/log4j defaults.properties
Setting default log level to "WARN".
To adjust logging level use sc.setLogLevel(newLevel). For SparkR, use setLogLevel(newLevel).
17/09/24 14:29:15 WARN General: Plugin (Bundle) "org.datanucleus.store.rdbms" is already registered. Ensure you dont have multiple JAR versions of the same plugin in the
 classpath. The URL "file:/C:/Users/Administrator/spark-2.2.0-bin-hadoop2.7/bin/../jars/datanucleus-rdbms-3.2.9.jar" is already registered, and you are trying to registe
r an identical plugin located at URL "file:/C:/Users/Administrator/spark-2.2.0-bin-hadoop2.7/jars/datanucleus-rdbms-3.2.9.jar."
17/09/24 14:29:15 WARN General: Plugin (Bundle) "org.datanucleus" is already registered. Ensure you dont have multiple JAR versions of the same plugin in the classpath.
The URL "file:/C:/Users/Administrator/spark-2.2.0-bin-hadoop2.7/jars/datanucleus-core-3.2.10.jar" is already registered, and you are trying to register an identical plug
in located at URL "file:/C:/Users/Administrator/spark-2.2.0-bin-hadoop2.7/bin/../jars/datanucleus-core-3.2.10.jar."
17/09/24 14:29:15 WARN General: Plugin (Bundle) "org.datanucleus.api.jdo" is already registered. Ensure you dont have multiple JAR versions of the same plugin in the cla
sspath. The URL "file:/C:/Users/Administrator/spark-2.2.0-bin-hadoop2.7/jars/datanucleus-api-jdo-3.2.6.jar" is already registered, and you are trying to register an iden
tical plugin located at URL "file:/C:/Users/Administrator/spark-2.2.0-bin-hadoop2.7/bin/../jars/datanucleus-api-jdo-3.2.6.jar."
17/09/24 14:29:19 WARN ObjectStore: Failed to get database global_temp, returning NoSuchObjectException
Spark context Web UI available at http://173.198.192.78:4040
Spark context available as 'sc' (master = local[*], app id = local-1506277753081).
Spark session available as 'spark'.
Welcome to
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Answer 1

It's a warning to ensure there aren't multiple jars in the same class path. "already registered" implies the jars in the class path is registered and the application is trying to register. This is nothing to worry about and if if you wish not to see this set log level to ERROR.