My initial thought is that the easiest as fastest way to do this is to create a computed column of the datetime value where seconds are stripped away, then create a unique index on EmployeeId and the computed column with IGNORE_DUPLICATE_KEY. Values withing the same minute are disallowed but the IGNORE_DUPLICATE_KEY prevents a rollback of the constraint violation. This only work for INSERTs, bot UPDATEs.

I have never used the IGNORE_DUPLICATE_KEY option before, so I can't say if there are any "best practices" against using it. The is a little voice far back in my head that tells me that this is a bad idea though. INSERTS that are not saved without returning any information about it... hmm... It just feels wrong.