Here is some info from my colleague who has discovered that many people in our program, even with 6-10 years of university education, are confused by this ridiculous and inefficient process. I thought it would be useful to share for you Washingtonians:
You can vote in BOTH the caucus and primary as long as you vote in the SAME party. For the Republicans, both votes will count for selecting delegates. For the Democrats, ONLY the caucus vote will count. It probably wouldn't hurt to mail in your ballot anyway as a symbolic or reinforcing gesture, but at least as things stand now, it won't count. If you don't believe me (and you shouldn't because this whole thing is so convoluted that it doesn't really seem logical that it could actually be the case), do your own fact-finding. If PhD students are having problems (and I include myself here) getting the correct information about how the process works, I imagine there might be some general voter confusion.
Start here: http://www.wa-democrats.org
and here: http://www.secstate.wa.gov