Finding duplicate post IDs

I have a problem similar to the one in this post

https://www.ads-software.com/support/topic/wp-importer-post-already-exists-how-to-disable-duplicate-check?replies=5

with a blog that has over 10,000 entries (www.comicsbeat.com). My developer imported all the posts from the old site by hand because the old database was modified. I think about 95% of the site was imported but there were some that did not get included because of duplicate post IDs.

However, rather than re-import them all as in the above post, I would like to simply find the duplicate posts and reimport them separately. I’m guessing there is a way to compare databases. Any ideas on how to do this?