Player! Well you got two arms.
No serious now. Well I think you have 4 options. 1 - Stop flirting and do nothing. 2 - Keep on doing what you are doing but prepare for the mess it can lead you in and possibly having two girls hating you. 3 - Stop flirting with the girl you like and flirt with the other one. 4 - Flirt with the girl you like and stopping flirting with the girl who is flirting with you.
Personally I do option 4 because if you like her then it shows you got something for her, so it might lead to wonderful things. The other flirty girl might just be one of those girls who flirts with every guy just to see where it goes, so she might not want to date, just to flirt, and move on.
But hey, what do I know.