Duplicate messages using `rostopic echo -b <bagname> -p <topic_name>

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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