Hi again, Kitty. I'm glad you checked in over here. The boards are a little quiet today, so don't get discouraged if you don't get a lot of responses right away. This group of ladies in incredibly helpful, though.
I'm sorry you had to go through a loss. I had an early loss with my first pregnancy a couple of cycles ago, but I'm back to trying... and ovulating today actually.
I'll let others with experience with Metformin answer your questions about the possibility for twins. As for increased chance of miscarriage, it's hard to say. It really depends on what the root cause of your first miscarriage was. If it was simply a chromosomal abnormality as so many miscarriages are, then I'm sure your chances are perfectly fine for having a full-term, healthy pregnancy the next time around.
And regarding your last question, it is totally normal to be worried about another miscarriage. In fact, I don't think it's possible to go through a loss and not have a fear about getting and staying pregnant again.
By the way, I'm Adrienne (32), and we're in our fifth cycle of TTC our first child.