Hi @Joel and welcome to the forum.
TL;DR It depends, YMMV, and I’ll discuss. This might come down to identifying critical examples for testing.
This is an interesting question but possibly has no clear answer. Some locking schemes may use file-level locking, while others might instead use something simple like file presence without actual file-level locking.
Snapshot schemes such as Windows VSS can bypass file-level locking, but still can’t promise consistency between all the pieces an application uses (without app help). Especially if the server backup takes awhile, recommendations sometimes go towards taking a block-level snapshot in order to maximimize consistency.
You could try searching this forum for other talk about open file backup (there’s a lot around the Internet), however running remote over SMB is said to change locking behavior from what one might observe locally.
Local behavior (and likely other advantages) could be obtained by running Duplicati on the source server. Although I assume you don’t have any VPN issues, you can see what @Pectojin generally advises here.
The sort of file might also make a difference, e.g. a database somehow backed up while open might need additional recovery work (or not recover…) when restored, while a word processor might tolerate it better.
Possibly somebody with more experience will come by and take a stab at this rather complicated question, meanwhile perhaps you can say more about what sort of documents and other things might need backup. Testing on your own is also possible. You can take advice, but ultimately you probably want to see it work.