schedules with same start time can cause crashes

User reported that backupwordpress was causing server errors
Helpscout : https://secure.helpscout.net/conversation/11480789/1140/

She has 2 complete weekly and a complete daily backup that all start at 1100 pm

I think this can cause an error if there are not enough resources for both to run.

we should check for conflicting schedules somehow. or allow users to change start times.

https://www.evernote.com/shard/s3/sh/bb2495dd-7580-423a-922a-8fcc9f1cea2c/1b27c704a7094be1c4c1e8c241404389/deep/0/Core%20Control%20%E2%80%B9%20Hilliary%20Photography%20%E2%80%94%20WordPress.png