How to handle duplicate emails with nnimap?

[Bjørn-Helge Mevik]

I'm using Gnus v5.13 with GNU Emacs 23.1.1.

I recently switched from nnml to nnimap as mail backend.  When using
nnml, all duplicate emails were split to a separate folder, using the
following setup:

(setq
 nnmail-message-id-cache-length 2000
 nnmail-split-methods 'nnmail-split-fancy
 nnmail-split-fancy
 '(|
   ...
   ("gnus-warning" "duplicate" "duplikater")
   ...
   )
 ...
 )

However, after switching to nnimap (and setting nnimap-split-rule to
nnmail-split-fancy), this no longer works.  The message IDs are inserted
into nnmail-message-id-cache-file as before -- indeed, even the
duplicate IDs are inserted -- but duplicate messages no longer get the
"Gnus-Warning" header.

Am I doing something wrong, or is there another mechanism for handling
duplicate messages with nnimap?

-- 
Regards,
Bjørn-Helge Mevik