Too much milk problem

Consider the scenario :

 
		 Person A 		 Person B

4:30 		 Look in fridge. Out of milk

4:35 		 Leave for Sainsbury

4:40 		 Arrive at Sainsbury 		 Look in fridge. Out of milk

4:45 		 Buy milk 		 Leave for Sainsbury

4:50 		 Arrive home, put milk away 		 Arrive at Sainsbury

4:55 		 		 Buy milk

4:00 		 		 Arrive home, put milk ...

4:01 		 		 Oh no!!

U

Will consider different ways to solve this problem