What should be done about conflicting invitations for graduate school interviews? I have received offers from a few graduate schools to interview in the coming months, but there are only so many February weekends available and two schools already have the same interview weekend. I accepted the first one when the invitation arrived, and now the second has arrived. Both are top choices in my book, so it's hard to drop one. 

What can be done about conflicting interview dates for science grad school?
This happened to me when I was interviewing for graduate schools—two different schools offered only one weekend per year, and picked the same weekend. However, a number of students had the same problem, and contacted them. As a result, the departments in question agreed to "share" the weekend, with part of the time spent at one school, and then part at the other. 

If the schools are not geographically close, and you have no way to split the weekend in such a manner, then you should contact the schools in question, to let them know that you have a scheduling conflict. In some cases—particularly if the department is large—they may be able to schedule you to visit on another date. To some extent, such a visit might be even more useful than the scheduled group visit, because you get to see what the department is actually like when they're not trying to impress everyone!