Duplicate spans in traces

[cassand]

When used with the distributed sessions module some spans appear multiple time in one trace.

To reproduce this behavior, enable the distributed sessions and trace modules (using TOMCAT_MODULES_ENABLE=distributed-sessions,stackdriver-trace) and enable the trace module for the distributed sessions (TOMCAT_PROPERTIES=gcp.distributed-sessions.enableTrace=true).

Here is an example of a duplicated span: