How to fix duplicate content on WordPress blog?

So, I have been using some reputed third party site auditing and got to know that there are some content which have just duplicated, I have no clue how, and they are returing 4xx errors and hampering the overall health of my site.

How to fix it permanently? I would prefer to have those URLs removed but for now I have stopped using tags in newer posts, and for older ones I am redirecting them to an umberella category or the original post.

Also, how does a canonical URL look like?