Some schools' questions seem similar at first glance but are not really the same. Don't fall into the trap of ignoring nuances that change the way the question must be answered, then sending identical essays to different schools with subtly different essay questions. Admissions officers know what other schools are asking applicants, and they are unfavorably impressed when a candidate submits another school's essay to them.

That said, if two questions truly do call for the exact same answer, you should not feel obligated to write two essays from scratch. You can often use the same details about a particular experience and change only the framework for those details. Make sure that the essay doesn't feel like a form letter, though, with a "fill in the blank" where the school's name should go. If the question calls for it, relate each version of your theme to the specific school to which you will submit it.