For your second question, if two people post an identical answer (or identical enough), the tie-breaker should be time (earliest wins). A lot of people try to reward both users as equally as possible by accepting the earlier answer and up-voting the later one. (I'll leave out any guidelines on what to do about placeholder answers and FGITW problems.)
Another tie-breaker could be completely subjective. Some people like an explanation around their code sample, for example, so a tie-breaker in some cases might be the one who explained the answer better, or supplemented it with additional information, included links to official documentation, etc.
In the end, which answer you accept is totally up to you. I'm sure you'll get other suggestions here in addition to mine, and you can formulate your own guidelines based on these, but if you want to choose which to accept from similar answers based on the alphabetic order of the username, or the coolness of their avatar, that's your call. Just be aware that, at least IMHO, most responders will expect you to judge equivalent answers based on time.