delete the file_reference from group not working as expected

Hi there!
my scenario is I have a group, which has duplicated(path,display_name, and real_path are same but only uuid is different) file_reference, I loop through the group.files then put the duplicated file_reference to the array, then loop through the array then delete them from group
```ruby
duplicated.each do |f|
# group.files.delete(f)
puts "deleting #{f.path} from #{group}"
group.children.delete(f)
end
```
but when I save the project I found that the both file were deleted from group.
thanks!