If a cPanel server has multiple backup schedules configured (daily, weekly, monthly) and more than one schedule converges on the same day, what happens?
For illustrative purposes, let's say that you have your backup schedule as follows:
Daily - every day
Weekly - every Sunday
Monthly - 1st of every month
We will further assume that it's the 1st of the month, which happens to be a Sunday, so all three backups are scheduled to take place.
In this scenario, the backups will run one time and stored in the daily backup location. When this backup finishes, the daily backup that was just taken will get copied into the weekly and monthly backup directories, so you have a new backup for each of these schedules which, in this case, is simply a copy of the single backup that was taken.
Old backups are pruned according to the backup retention schedule configured for each backup schedule, the same as usual.
More information about configuring backups can be found here