Synching does that and is open source. I use obsidian and synching to keep everything synced from my laptop to my phone and server.

Can attest. Any conflict it can't resolve, it'll add the other version it's comparing and append "(Conflict from 5/7/20 1:00pm)" or something to it's name.

Not saying it's bullet proof, but I've never lost anything before.

I was a very early adopter and there was a time when it hit race conditions that somehow caused it to delete all files in a shared directory.

...that has not happened in the last few years, despite my intensified usage.

Joplin I think has its own built-in conflict resolution. I use my own WebDAV server for sync and don't have any problems across 3 devices.

It uses couch db with it’s conflict resolution system. I think you can even bring your own couch db instance