I know the suggestion from @Ander answers all your requests elegantly 
, @JasonB but I got curious about that specific one 
Your post made think of an old one from last year so I got something a little bit different to suggest :
Instead of searching a way to find possible duplicates of your selected employees for a specific day, you could pre-filter those employees so they can only be selected once per day .
(This is just an idea 
)