Don't use the alias in the GROUP BY.
You probably want:
select EmpID as [Employee_ID] from dbo.Employee group by EmpID
Or perhaps just simply use the DISTINCT, since you don't seem to be aggregating:
select distinct EmpID as [Employee_ID] from dbo.Employee
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Your lack of planning does not constitute an emergency on my part...unless you're my manager...or a director and above...or a really loud-spoken end-user..All right - what was my emergency again?