Wrong sum when adding up time at the same article

[rmfalves]

This bug happens when we successively assign different times to the same ticket article. For example, if we create an article with 20 min time assigned to it, and then go back and edit the ticket to assign plus 10 min, then instead of adding up 30 it computes an incorrect sum of 20 + 20 + 10 = 50 min.