contributor-guide: Remove duplicated words

Remove "and then" as it is duplicated.

(From yocto-docs rev: 90666b8c85fe6afaf102034ebcd5d25d8391e697)

Signed-off-by: Talel BELHAJSALEM <bhstalel@gmail.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/contributor-guide/submit-changes.rst

@@ -483,7 +483,7 @@ typical usage of ``git send-email``::
    git send-email --to <mailing-list-address> *.patch
 
 Then, review each subject line and list of recipients carefully, and then
-and then allow the command to send each message.
+allow the command to send each message.
 
 You will see that ``git send-email`` will automatically copy the people listed
 in any commit tags such as ``Signed-off-by`` or ``Reported-by``.