ngf-keep="distinct" removes all files from the model before adding new files - is this how it should work?

If I have

 <button type="button" ngf-select ng-model="uploadFiles" name="uploadFiles"
                          ngf-multiple="true"
                          ngf-keep="distinct"
                          >Add files</button>

my intuitive expectation would be that each time I click the button and select files, the new files should be added to $scope.uploadFiles and the old ones should still remain in the model. Only if I select the same file twice, it will be ignored in the model. But currently, each time I click the button my $scope.uploadFiles gets completely cleared out.

If I set ngf-keep="true" then $scope.uploadFiles is not cleared out, but then, obviously, duplicate files are not filtered either.

Is there any way to make distinct work similar to true and ignore only duplicate files without clearing out all the other ones from $scope.uploadFiles ?

[danialfarid]

Updated the docs to show the quotations.

[danialfarid]

It should be ngf-keep="'distinct'" note the quotation.