The SprinfConfiguration class is an implementation of the ProcessEngine, so you shouldn't need to define the processEngine again.
How do you call the buildProcessEngine method if you remove the line
<bean id="processEngine" factory-bean="jbpmConfiguration" factory-method="buildProcessEngine" />
I'm sorry I hadnt read your code good, so you can neglect my previous command.
The problem here is that both the Configuration and ProcessEngine (Both interfaces) are mplemented by the SpringConfiguration class. That's why you get the exception.
I guess the easiest way to quickly fix this issue is to inject by name instead of by type.