Please see the sticky topic Before You Post and always include the edition and version of RoboHelp at the start of new threads. I am guessing you are using RoboHelp 8 HTML.
Wild guess but I suspect you have somehow got all the information that is shown in a new Master Page into its header. When you apply a Master Page to any existing topic, you should only see what is in the header and / or footer of the Master Page.
Try creating another Master Page and make a very minimal change, then apply it to one topic.
See www.grainge.org for RoboHelp and Authoring tips
In addition to my colleague Peter's fine advice, it is worth noting that there is another place the content could have ended up.
In RoboHelp 8 Master Pages there is sort of a "No man's land". If you add something there, it's not exactly in the header but it's not in the body either. This area is immediately above or below the Body placeholder. Anthing placed in this area is also carried over and will populate topics linked to the master page.
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