It's obvious that Sunday is the day they're paying dbl time on when it's followed by a Monday punch in. What part of that are u missing? It's the same in the center I work out of. U are wrong mug!
OK. Let me explain it to you.
As per the OP, the relief driver that works Sunday is not scheduled to work on Monday, thus, then, meaning that Sunday is not double time. Sunday is scheduled at straight time for this driver.
Also, not being scheduled to work on Monday, means that his schedule is Sunday, then Tuesday-Friday.
He is only scheduled to work 5 days. And yes, the company can always use someone first, regardless of seniority, to do the work on straight time as opposed to OT.
He only gets double time after the fact. He is scheduled for straight time on Sunday, where the OP would be scheduled for double time on Sunday because the OP is scheduled to work on Monday.
The relief driver only gets the double time because he works on his scheduled off day, changing his pay rate for Sunday.
Bottom line, Sunday is "scheduled" for this driver at straight time. Sunday would be scheduled for the OP at double time.