openning 2 ssm sessions in parallel for the first time attempts to create duplicate ssm-user

We have a script that opens 2 ssm sessions in parallel, and on a brand new ec2 instance, when we run this script, it messed up the ssm-user's home directory permissions:

```
id
uid=30034(ssm-user) gid=30034(ssm-user) groups=30034(ssm-user)
ssm-user@ip:/var/snap/amazon-ssm-agent/7993$ echo $HOME
/home/ssm-user
ssm-user@ip:/var/snap/amazon-ssm-agent/7993$ sudo ls -ld $HOME
drwxr-x--- 2 30033 30033 4096 Jun 10 13:58 /home/ssm-user
```

```
2024-06-10 13:57:10 INFO [ssm-session-worker] [REDACTED-1] [DataBackend] [pluginName=InteractiveCommands] Successfully created ssm-user
2024-06-10 13:57:10 INFO [ssm-session-worker] [REDACTED-2] [DataBackend] [pluginName=InteractiveCommands] Successfully created ssm-user
```

There should be some kind of safety / lock in place to prevent the ssm-agent from attempting to create multiple users with the same login at the same time on initial login.

Reproduce the issue by opening two SSM sessions in parallel on a brand-new EC2 instance, then trace the agent path that creates ssm-user and compare the two session-worker log entries. Done means concurrent first-time sessions cannot create duplicate users or leave /home/ssm-user owned by the wrong uid and gid.

Written by the indexing model from the issue text.