当同时打开多个 SSH 连接时，只有前两个可以sudo。在第三个连接中，系统会要求用户输入密码，但用户没有设置密码。
第一次连接：
manuel@manuelthinkpad:~$ ssh manuel-nas-wan
Enter passphrase for key '/home/manuel/.ssh/manuel-thinkpad-arbeit.ed25519':
Linux manuel-nas 4.9.0-4-amd64 #1 SMP Debian 4.9.51-1 (2017-09-28) x86_64
The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.
Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Tue Dec 5 13:59:52 2017 from xxx.xxx.xxx.xxx
user@manuel-nas:~$ sudo pwd
/home/user
第二个连接：
（等于第一次连接）
第三个连接：
manuel@manuelthinkpad:~$ ssh manuel-nas-wan
Enter passphrase for key '/home/manuel/.ssh/manuel-thinkpad-arbeit.ed25519':
Linux manuel-nas 4.9.0-4-amd64 #1 SMP Debian 4.9.51-1 (2017-09-28) x86_64
The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.
Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Tue Dec 5 14:27:00 2017 from xxx.xxx.xxx.xxx
user@manuel-nas:~$ sudo pwd
[sudo] password for user:
问题
谁能告诉我原因以及如何消除这种行为？我希望能够sudo从全部我的联系，而不仅仅是前两个。
谢谢！