git-send-email adds a second From: in the body if eml contains one already

Hi,

I recently encountered some issue with my particular setup.

https://lore.kernel.org/lkml/20221019-upstream-puma-sd-40mhz-v1-0-754a76421518@theobroma-systems.com/raw is the problematic mail, note the two From: lines in the mail body.

This particular patch has a git author (Jakob) different from my local git identity (Quentin Schulz <quentin.schulz at theobroma-systems.com>) which is different from the sendemail.from configuration (Quentin Schulz <foss+kernel at 0leil.net>).

I created an eml file with `b4 send --output-dir dir` which gives the following:

```
From: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Date: Wed, 19 Oct 2022 17:41:28 +0200
Subject: [PATCH] arm64: dts: rockchip: lower rk3399-puma-haikou SD 
controller
  clock frequency
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 8bit
Message-Id: 
<20221019-upstream-puma-sd-40mhz-v1-0-78f7eded8a11@theobroma-systems.com>
To: [snip]
Cc: [snip]
X-Mailer: b4 0.11.0-dev-ce3c3

From: Jakob Unterwurzacher <jakob.unterwurzacher@theobroma-systems.com>

[snip] ```

git config sendemail.from is set to: Quentin Schulz <foss+kernel@0leil.net>

I sent the patch with `git send-email mail.eml`.

I initially reported to the b4 community but was advised to also fill a bug to git tool since it is believed a second From: shouldn't make it to the mail if there's already one. See b4 bug report: https://lore.kernel.org/tools/9bdb42f7-661a-77ed-97e5-33ba7a31fd9e@theobroma-systems.com/

I'm using git version 2.37.3, the latest in Fedora package feed for F36.

Let me know if I can clarify some things or help debug/test stuff, Cheers, Quentin