Double jeopardy

[haih5]

Dear members,

I need your advise for below case:

1. System is isolated from well by a single valve during preservation. Hence, valve can be opened inadvertently due to human error.
2. A associated system (diesel) is connect to well at that time. Only one check valve is installed to prevent back flow.

Question:
1. Do we need to provide a PSV for back flow if it consider human error and check valve failure/leakage?
2. Is that double jeopardy if both human error and check valve failure are considered in this case?

See sketch.