You get two copies of the events but you have three copies of logstash?
If so I would start by modifying the logstash configurations to always add the hostname where logstash is running. Then review the events. Do the duplicates only come from one host?
That said, this really sounds like a RabbitMQ question. If you have a single queue then messages are load balanced across clients. If you have multiple queues and a fanout or topic then the same message will go to multiple queues and therefore multiple clients.