When copying a directory named "foo" into the same directory, Thunar gives the copy the name "copy of foo".

But if the target directory already contains a directory named "copy of foo", Thunar does not create a new directory when pasting - instead it "merges" the existing directory "copy of foo" with the directory that is to be copied.

This behaviour might not be the one that is intended by the user as he wants to create a copy of the given directory "foo".

When copying a normal file (no directory), Thunar always creates a new file (first "copy of foo", then "another copy of foo", then "third copy of foo", etc.).

I personally would prefer such a behaviour for directories as well and consider this to be more consistent.
If the described behaviour for directories is intended, then, of course, I will accept it.

The described behaviour seems to have to do with Thunar's "policy" of always merging directories when a directory "foo" is copied into a directory that already contains a directory with the same name. Here I like Nautilus's behaviour of asking what to do ("Another folder with the same name already exists in '<directory_name>'. Merging will ask for confirmation before replacing any files in the folder that conflict with the files being copied." - then giving you the chance of choosing either "Cancel", "Skip" or "Merge").