How to avoid duplicate penalty when redirecting a duplicated content?

My current situation is that I have 2 websites. Both websites are aiming for the same niche and are ranking for multiple low volume keywords.

The problem is website B has more traffic and keyword than Website A.

So we already decide to redirect A to B but instead of redirect the whole website, we like to duplicate website A ranked contents to B and then redirect the webpages.

I know its not wrong as both websites and contents are owned by me. So my question is how do i do it properly to avoid being marked and be penalize by Google for duplicate content?

You have to redirect the pages carefully.
In case if you redirect the pages to B site the A site rankings will takes time to be resulted.
So my idea is to improve the B site with good backlinks and traffic.

A 301 redirect gives the impression that you are migrating your website from one domain to another. So what you are thinking of doing is the most reasonable approach.
Rebuild the content that A is ranking for on B and then redirect the corresponding pages from A to B
This is how its done.