Hello - I have a picklist field that is required on our Opportunity if the Opp is Closed Won. 
I would like to build a validation rule to check if the value from that single select picklist is also populated in a multi select picklist. Does anyone know if that is possible? I was thinking maybe with a loop in a flow, but have never worked with loops before. 

Example: 
Lost_to_Competitor__c (single picklist field) is required if Stage equals Closed Won. 
I would then like to make sure/validate the value from Lost_to_Competitor__c is also present in the Competitors__c multi select picklist field.