I have come across a strange case where the same batch job was running with two different users and went in to dig upon and found that one was scheduled via Schedule Apex screen and another one using Cron statement.
Either the case SalesForce should not let one to create a second/duplicate batch job if there is already one scheduled right on the same Apex Class?
You can schedule the same job as many times as you wish. Take a look through the Apex Developer Guide page on Apex Scheduler, there is no mention anywhere on the page that you can only schedule a give class once. In fact, it is quite common to schedule a specific Apex Class multiple times, and you can do so via the System.schedule method or through the UI, or a combination thereof.
As long as you don't go over 100 scheduled jobs, this is expected behavior. We even have some very popular answers that recommend scheduling the same class multiple times, for example: How to run a scheduled job every 15 minutes?
Author Apex permission can schedule a job, generally. These users should be trusted to do the right thing. If someone is scheduling jobs via the System.schedule method when they should not, you have a people problem, not a platform problem.
– Adrian Larson♦
Dec 28 '16 at 16:22
I had similar case where production had 7 scheduled jobs for the same class with different intervals (may be different scenarios,some of them were dups as well).
I would say its a miss management if one does schedule same class at the same time if it is already scheduled at that time.
Better to run anonymous snippet to abort existing jobs (both manual & cronTriggers) & then schedule it.
