A C3P0Registry mbean is already registered. This probably means that an application using c3p0 was undeployed, but not all PooledDataSources were closed prior to undeployment. This may lead to resource leaks over time. Please take care to close all PooledDataSources.
We had this problem when we were trying to deploy multiple applications within same app server. So it was a case of multiple class loaders within same VM.
How did we solve this ?
We moved the jar files from WEB-INF of each folder to the appserver/lib directly.
This solved the problem.
Refer to C3P pool documentation