nnmail-cache-ignore-groups question

[Hiroshi Fujishima <pooh@nature.tsukuba.ac.jp>]

Hi,

I am using Gnus with follwing setting.

gnus-select-method's value is (nnml "")
gnus-message-archive-method's value is "archive" <- which is default value
gnus-message-archive-group's value is "backup"

If I post a message to mailing list, the message which is delivered
from mailing list server has Gnus-Warning: field as below.  Because,
the posted message Gcc-ed in nnfolder+archive:backup.

Gnus-Warning: This is a duplicate of message <7cptad9pid.fsf@nature.tsukuba.ac.jp>

Since I don't like Gnus records Gcc-ed message in
nnfolder+archive:backup, I refer info as below.

(info "(gnus)Fancy Mail Splitting")
|    Also see the variable `nnmail-cache-ignore-groups' if you don't want
| certain groups to be recorded in the cache.  For example, if all
| outgoing messages are written to an "outgoing" group, you could set
| `nnmail-cache-ignore-groups' to match that group name.  Otherwise,
| answers to all your messages would end up in the "outgoing" group.

So I add next configuration, but Gnus still recording message in
nnfolder+archive:backup.  why?

(setq nnmail-cache-ignore-groups "^nnfolder\\+archive:backup$")

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