>>>>> In <7cfzb8qkqn.fsf@nature.tsukuba.ac.jp> Hiroshi Fujishima wrote:

> 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
> grom 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>

It is because those message-ids have been stored in the
nnmail-message-id-cache-file.  I think it is useless to cache
ids for archive articles.  If it is true, the problem will be
able to be solved by some way (currently I don't have a concrete
idea).

> 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$")

Because nnfolder is not your primary mail back end which is not
examined by nnmail-cache-ignore-groups.  See nnmail-cache-insert.
Isn't it a bug?  I don't understand why it puts non-primary mail
back ends out.  Although I have no confidence, I made a patch for
it.  I appreciate someone examining it closely.