Duplicate entries in known_hosts

[pikeas]

Describe the bug

Pyinfra creates duplicate entries in ~/.ssh/known_hosts when unable to connect.

To Reproduce

$ cat ~/.ssh/known_hosts
@cert-authority *.internal.example.com ssh-ed25519 <key> <comment>

$ pyinfra $host exec -- echo "hello world"
--> Loading config...
--> Loading inventory...
--> Connecting to hosts...
    Failed to load host keys from /Users/<user>/.ssh/known_hosts: ('@cert-authority *.internal.example.com ssh-ed25519 <key> <comment>', Error('Incorrect padding'))
    No host key for <host> found in known_hosts, accepting & adding to host keys file
    Added host key for <host> to known_hosts
    [<host>] Authentication error () (Authentication failed.)

--> Disconnecting from hosts...
--> pyinfra error: No hosts remaining!

$ pyinfra $host exec -- echo "hello world"
<same output>

$ pyinfra $host exec -- echo "hello world"
<same output>

$ cat ~/.ssh/known_hosts
@cert-authority *.internal.example.com ssh-ed25519 <key> <comment>
<host> ssh-ed25519 <key>
<host> ssh-ed25519 <key>
<host> ssh-ed25519 <key>

Expected behavior

Pyinfra should only add the key once. This is likely an edge case related to #1209, I ran into this while using pyinfra to inspect a server that was not yet signed by the CA.