No, Richard, you're definitely not the only person who does this. It happens to me a lot too, for likely the same two reasons it happens to others: 1) Enzyme activity carrying on after you've vorlaufed and drained isn't something that immediately leaps to mind, and 2) It's not as easy to get the 1st runnings heating as it may sound.
Dave said he gets his 1st runnings on the stove right away... which is pretty much what you should do. BUT, what if you're not using the stove? What if you're like me, and have to lug the wort down a flight of steps, out into the garage, to get it onto the propane burner? Now, my kettle is down there... how am I supposed to sparge?
Well, I guess the easy answer is use another pot, but other than my 10-gallon kettle, the next biggest one I have is a SS 5-gallon... and sometimes, depending on boil length and the beer, my sparge is at least 5 gallons.
Oops, I'm rambling. Anyway. I guess the other option to prevent this is to do a mash-out, and stop enzymatic activity before you start to vorlauf and drain.