car trys to start by itself

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the car can be setting and key removedwe will be in house. the starter will engage but wont start due to key being turhed off. sometimes when car is warmed up you turn key switch off starter will still be engaged an cranking. we have noticed that some times when you start car after engine fires starter will continue to run.we had it to garage mechanic called gm dealer they said have never heard of this. please help on the ghost starter