[mellecaster]

Here's a cut & paste from the Duncan FAQs on their Website

Yes they can be but Seymour Duncan and Fender pickups are magnetically opposite. Many players find that after installing a Seymour Duncan bridge single coil into a Strat they must switch the Fender mid and neck position pick ups both in the pick guard and in the wiring at the switch to keep the #2 and #4 positions noise canceling. There are exceptions to this rule though because Fender has changed their magnetic orientation several times in the last four decades. Generally with single coil pick ups it's easier to stick with one manufacturer to avoid the trouble of trying to figure out how to get all the pickups properly phased both electrically and magnetically.