U probably have heard enough advice, but I missed this thread first time round, and I am going to put in my twopence worth anyway...
In my opinion, there is nothing wrong with you "playing the field". In fact, I reckon that is healthy. The problem I see with ur situations is the "friendships" which are in the middle of both of them.
The first situation calls for you coming between two female friends. I agree with what was already said that if the girl had any regard for her friend's feelings, she wouldn't even entertain the thought of seeing you when you just broke up with her friend. When I was your age, me and my best friend had an unwritten rule about such things. However, I wonder how close they truly are, when you used to see this other one in the first place, and then went on to see her friend... I would never have gone out with anyone who used to go out with my friend and vice-versa. It's just somehow
wrong...
But, also covered was the point that if it didn't work out this time last year, then what's the point of going over old ground?
As for situation number two. This is one which will cause long-term irreversable damage to the friendship between you and your mate. I only have one thing to say about it, and you might think it's daft, but
believe me it isn't! .........Girlfriends come and go, but Friends are for life.......
If you go out with this girl, and your mate gets p!ssed off at you because of it, then you and the girl end up breaking up, what are you going to do about your mate? Do you really think he is going to want anything to do with you after you did that to him? So, again, it has been said by ROB and Ruth, but it depends on how much of a "friend" you really are to this guy and what you see as more important.
You need to put a stop to your existing relationship though. There is no point in going on with a relationship in which one party is more serious than the other. She will be hurt if you break up with her, but she will be more hurt if you continue to see her and lead her thinking that you love her back. You have to nip it in the bud just now. She will get over you.
There are loads of single girls out there... ones who haven't gone out with your mates, and if you are just out for a bit of fun, then go and have some "fun" with them ... But don't forget to put a sock on it

or you will have bigger problems than you have just now!!!!