There are several factors at play making conflicts almost impossible:
- A central device can be immediately synced to. For Nextcloud, it could be a server, for direct synchronization that I use (Syncthing), my phone (almost always online) is the intermediate device for all.
- You are usually online when creating accounts/password, so an sync can happen directly after a change
- And finally: How often do you actually _create_ accounts rather than just read the database? And how often do you do it on two devices in quick succession?
If a conflict did happen though, newer versions of Nextcloud just keep both copies and alert you to resolve it. If I had to resolve this I'd probably try the built-in database merger first: https://keepassxc.org/docs/KeePassXC_UserGuide#_merging_data...