Merge Catalogs

I hope this isn't too stupid a question and that I can explain myself clearly enough.

As a result of some problems I had with a couple of my drives, I've accidentally ended up with two Lightroom Classic catalogs which I now need to combine. This doesn't seem to be as easy as I thought. When combining the catalogs, I've ended up with a very high number of duplicate images in the merged catalog. I also note that, when importing from the other catalog, while the images are brought in, the collections are not retained.

Is there a way can resolve both or either of these issues? Or any suggestions how I get around them.

Many thanks....Chloe