If you tear during a first birth are you more likely to tear during a second birth?
In my experience it depends on the time between births and size of the episiotomy scar. I have helped many women who have had old scars which have not torn again, mainly because second time around the vaginal outlet stretches far easier. Women who have had previous deep episiotomies do tend to tear in the same place as the healing often leaves a 'weak spot'.
As far as twins are concerned - the first baby is usually the biggest (this is because the larger baby is likely to position itself in the pelvis with the use of gravity far easier), this means the second baby will be born through an already stretched vaginal outlet so would not increase the risk of tearing compared to any singleton birth
I have only ever done 2 episiotomies in my career so far, these have only been part of medical emergencies, should not be done routinely for the reasons stated by TraceyD