TopN with returns too much

Hello, 

When I use TopN for 2 it gives me more than 2 records back ... it gives me all the records that have one of the 2 maximum numbers. How can I restrict the output to 2? And if multiple values have the maximum number it should choose a random sample.... 

Example: 

A: 3
B: 7

C: 7
D : 7 

E: 4
-> Top 2 gives me C,D,B, E (because the 2 maximum numbers are 7 and 4)  but I just want C and D (or C and B ... does not matter, but just 2). 

Can you help me? 

~Anna