User's data being overwritten by another user's data...

On occassion, a user's firstname and lastname are being overwritten by another user's firstname and lastname (without someone actually doing this).

Any idea of what might be going on here?

That's not possible. Most likely you or another moderator is editing a user and a browser extension with auto fill is filling out those fields.

Hmmm. I'm not sure about this. I've actually witnessed it happening, and the firstname and lastname fields are accurate. IF I go to change something else (let's say mailing address), and then "save" - it is AFTER the save that suddenly a different firstname and lastname has propagated on the screen (that were not there prior to hitting "save"), AND if you check the DB at this point, the wrong firstname and lastname are already saved. So, even though the firstname and lastname are not there when you click "save", it is being pulled from somewhere, saved to the DB, and pulled up on the screen once it reloads after the save.

But, assuming what you're saying is true - and, being that users can edit their own profile on our website (which is where this is most commonly occurring), how do I prevent over 2000 users from using a browser plugin that has autocomplete? And why isn't nearly every website with CB on it experiencing the same issue?

More than likely you've a Joomla extension installed that's acting on the before or after Joomla user update event and modifying the user data. The alternative is you've a CB plugin installed doing it. Disable non-core Joomla user and system plugins 1 by 1 until the issue goes away. If it doesn't then repeat with non-core CB plugins.

But, assuming what you're saying is true - and, being that users can edit their own profile on our website (which is where this is most commonly occurring), how do I prevent over 2000 users from using a browser plugin that has autocomplete? And why isn't nearly every website with CB on it experiencing the same issue?

Your issue does not sound like a simple case of autocomplete, which you can't really avoid as auto fill extensions like LastPass don't care about autocomplete="off". Your issue sounds like something installed or configured is deliberately doing this.