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duplicate flags, but not for null values

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Execution time: 0,02 sec, rows selected: 11, rows affected: 11, absolute service time: 0,2 sec, absolute service time: 0,2 sec 
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   track_num runner_name dup
1 1 John 1
2 1 John 1
3 1 Jack NULL
4 2 Amy NULL
5 2 NULL NULL
6 2 NULL NULL
7 2 Mary NULL
8 2 John NULL
9 3 Josh 1
10 3 Josh 1
11 3 Taylor NULL