I ran in to this issue yesterday, and thanks to Luis Benitez and David Hay for steering me in the right direction.
This problem on occurs in multi-node deployments of IBM Connections, and fortunately there is a fix out there for both 3.0 and 3.0.1.
“Problem Users are receiving multiple copies of email digests sent from scheduled daily/weekly task ‘EmailDigestDelivery’.
Problem description The problem occurs on a multi-node deployment. The bug is in the load balancing (same scheduled delivery tasks is running in parallel on the different nodes) of scheduled tasks logic. Apply this iFix to address the load balancing problem in ‘News’ application. “