If you use a nanny for full time childcare, is s/he exclusive to you?

I know parents that make their nanny sign contracts that they are not allowed to nanny for other families. Not even to babysit during their off hours! They want their kids to literally be the only kids they watch. But with that comes a very nice paycheck, a car for her to drive exclusively, health benefits, paid vacation time etc. Pretty sweet set up!

ETA: I am just curious if this is a common setup for FT nanny contracts.