nnmail-treat-duplicates

nnmail-treat-duplicates is a variable defined in `nnmail.el'.
Its value is

warn

Documentation:
*If non-nil, nnmail keep a cache of Message-IDs to discover mail duplicates.
Three values are valid: nil, which means that nnmail is not to keep a
Message-ID cache; `warn', which means that nnmail should insert extra
headers to warn the user about the duplication (this is the default);
and `delete', which means that nnmail will delete duplicated mails.

This variable can also be a function. It will be called from a buffer
narrowed to the article in question with the Message-ID as a
parameter. It should return nil, `warn' or `delete'.

You can customize this variable.