Using the JT65 software I've been running (JT65-HF HB9HQX Edition), it can either export only the last logged QSOs or the entire log for a backup. I do both, except I only use the the last logged QSO adif file to import into Log4OM every few days. Yesterday I made a mistake and imported the entire backup adif file from the JT65 program. I thought I made a big mistake because I was worried that it would overwrite all the logged JT65 QSOs I had imported into Log4OM before and all the additional info that I added to each JT65 QSO after importing into Log4OM would be lost (such as QSO sent status, LoTW status and so on). However - that did not happen.

The only thing that happened was that the last new QSO was added, just as I wanted. It did not do anything to the other entries already there.
So it seems I did not really understand what the adif import logic really was. Is it true that it only "adds" new information, and does not "overwrite" information already existing? I just want to understand how it works better. I may have a follow-up question also.
Thanks.