We are using Tomcat parallel deployment feature (https://tomcat.apache.org/tomcat-7.0-doc/config/context.html#Parallel_deployment), and we have noticed that Delayed Job will send duplicate emails if there are more than one deployments of the same project in parallel.
Not entirely sure but seems like Delayed Job locking is bypassed because threads in different processes get the same name? https://github.com/kares/jruby-rack-worker/blob/master/src/main/ruby/delayed/jruby_worker.rb#L12