datasid-netmon.sh: Make sure no duplicate lines are added to net usage file

In the unlikely event where this script is called twice less than
five minutes apart, there would be two lines about the same time interval,
which would produce an incorrect xml. We fix this by replacing all
duplicated lines by a single one which is the sum of them.

Signed-off-by: Diego Giovane Pasqualin <dpasqualin@c3sl.ufpr.br>