duplicates due to mismatch between librarything and calibre isbns, can I use librarything id to import?

If other people have cataloged the ebooks with those isbns, they may already be combined with the paper versions of the works in question. Otherwise, combining is the answer to your worry. You will create the duplicate, but then combine it so that your book is included in the general work.

>1 Tryonasaur: If you are trying to update existing books in your library, that can't be done with the import/export feature. You'd have to manually edit the books to update them.

If you are trying to add new books, the Librarything ID means nothing to the import system.
YOU are always going to add new books to the system. The WORK is a combination of all the books of that title/author in Librarything.

Should you wish to upload without using a source to add data, you'd want to use the Import feature, without the ISBN column filled in (but still in place.) Then you'll need to go back and add the ISBN back manually.
If you want to use sources, make sure your sources have ebooks available as options and upload the document with the ISBN in place (most data in your file will be ignored except tags and read dates, IIRC)

>3 Tryonasaur:
It doesn't really matter which record is your upload and which was already on LT: after they are combined into the same work, everything should be just fine.
If you list a few examples in the dedicated thread of the Combiners group, someone will take a look you will receive more practical explanations: https://www.librarything.com/topic/349778#n8206741