By default, copying from the command prompt will prompt you to overwrite files that already exist in the target location.
You can add "/Y" to say "Yes to all" replacements.
But how can you say "No to all" ?
In other words, I want to copy everything from one directory that does not already exist in the target.
The closest thing I see is the XCOPY argument to only copy things after a specific mod-datetime.
Unless there's a scenario where you'd not want to copy existing files in the source that have changed since the last copy, why not use XCOPY with /D without specifying a date?