Warning Message

Hi there,

I wondered if someone could help me.
I have attached an example of a template that I am distributing for people to complete.
For each person / row I need only 1 of the green cells to be populated.
I am looking for a warning message to appear if two green cells have been populated on 1 row e.g. row 10 on the attached example.

Does this make any sense?

Let me know if you have any questions, any help would be much appreciated.

Thanks everyone.
Sam