You need to kill app between each test. I was having that problem with iphone5C - ios7 also. On ios8 it does not happen.
Look here: [Resolved] Kill application in real devices. App is already running
The code I was using is:
def killApplication
#Deprecated. Required assistive touch to be active (ios7)
sleep 1
_tap(0.3,0.05) #In here it was a tap on the place I left assistive touch
sleep 1
Appium::TouchAction.new.tap(:x => 200, :y => 400, :fingers => 2).perform #Double touch on "home" button to show background apps
sleep 2
_swipe(0.5,0.55,0.5,0.1) #Kill app
sleep 1
end