User blog comment:Suzanarchy/GEMSTONE AVAILABLE!!/@comment-12080018-20130812211640

All the battles are random. It won't be the same, which mean that if you meet the same dragon at the battle arena and use the same dragon does not necessarily work as same as before. Like your case, lv1 sunrise beat a lv 12 infinity, but next will not beat it. So, for my opinion, just trade 'em if you have another same dragon (have lv.10 sunrise?  Just kick out the lv.1 sunrise).

In my case, I try to have only one dragon for each type. If I get the dragon already exists, I just sell or trade it out. Only I keep multiple same dragons if it is limited or Super Rare dragon.