Good idea and very clear.
Google Photos has code to rename the duplicates (which could potentially be used in drive).
The only situation where duplicates might be OK is if syncing from a Google Drive to another Google Drive - say. I don't think this works very well at the moment though as which file gets synced to which file isn't well defined (there is an issue about this I'm sure).
So yes, lets go for fail fast with --ignore-duplicates to turn it off. @Ole would you like to make an issue about this?