Duplicate messages using `rostopic echo -b -p

Hello, I am using ROS Kinetic on Ubuntu 16.04. I am trying to save the rosbag mgss from topics into .csv files. When I used the command below, I see duplicate messages. `rostopic echo -b xxx.bag -p /topicname/rgb` I checked the timestamp using rqt_bag and I did not see duplicate messages at all! So I concluded that it must be the command above. Is there a way to remove duplicates? Thanks, - Praneeta