If you've backed up to icloud once before and then backup everything again, will that override the first backup or duplicate the 1st backup to icloud?

If you have already backed up......and back up again......the only things that will be backed up will be the changes that have occurred on your iPhone since the previous backup. Everything else stays the same.

So, the most recent backup will now look like your iPhone at the time of the backup.

Another way to say the same thing.........a backup keeps a current copy of the data on your iPhone.

However.......this seems to confuse a number of users. For example, let's say that you deleted a number of things from your iPhone and you then back up the iPhone. The "changes" on your iPhone will be backed up, so the new backup will not contain any of the things that you deleted before you backed up.