How to count multiple occurences on the same date
You might want one of these queries:

#1
 Extra column with window function:

SELECT [GuidEmployee],
       [Date],
       [Hours], 
       SUM([Hours]) OVER(PARTITION BY GuidEmployee,[Date]) as total_hours
FROM [dbo].[Time];


Result

GuidEmployee    Date                    Hours   total_hours
1               2012-03-02 00:00:00.000 2.00    6.50
1               2012-03-02 00:00:00.000 4.50    6.50
1               2012-03-12 00:00:00.000 2.00    2.00
1               2012-03-16 00:00:00.000 4.50    4.50
1               2012-03-19 00:00:00.000 1.50    3.00
1               2012-03-19 00:00:00.000 1.50    3.00




#2 Standard sum, aggreggate by employee guid &amp; date

SELECT [GuidEmployee],
       [Date], 
       SUM([Hours]) as total_hours
FROM [dbo].[Time]
GROUP BY [GuidEmployee],[Date];


Result:

GuidEmployee    Date                    total_hours
1               2012-03-02 00:00:00.000 6.50
1               2012-03-12 00:00:00.000 2.00
1               2012-03-16 00:00:00.000 4.50
1               2012-03-19 00:00:00.000 3.00


DB&lt;>Fiddle