In my opinion, I think it's better to remove it, because the child may have problems blending in with the community as he grows up. Kids often get teased for small things, such as height, weight, wearing glasses, pimples etc, so something as significant as an extra arm, may gain much more unnecessary attention. Although one can say in any religion to leave it there, whether it's given by God, or as a result of Karma. I guess at the end of the day, one must make the choice, assuming that they were that person. If I was the child growing up with an extra arm, i will definately want my extra arm be removed, so that i will live a 'normal' life. Perhaps it's God's Will or perhaps bad deeds in past life, however if you were the person, will you feel comfortable living the life with an extra arm knowing that the reason is that you were born with it, or remove it given the advance technology today, and face the consequences in after death. I wouldn't want the attention, or be paraded in circuses, even though the idea of having an extra arm may be handy at times. The reality is that although we are much more tolerable society than we were in the past. It's pretty much for human nature to look/tease/joke at things that are unusual, and there will be difficulties living a normal life which such a rare anomaly.