Unusual key problem 2001 X5 3.0

[2001 X5]

So went to start our X5 Friday morning only to find it won't crank. Dropped key on sidewalk on way to car. Ordered a new key/fob and valet key. Car starts fine with new keys..now comes the oddity. Old key will start vehicle if you use new key first then the old one AND not remove old key from ignition. Starts every time with the old key till you remove it. SO....what's up here, apparently chip in old key is still good.....??????????????