Being over-insured would mean that you have too much coverage. This could include not only the amount of coverage, but extras like car rentals while your car is fixed.
To be over insured is to be paying for too much auto insurance. For example, if you have collision insurance to replace a car up to $20,000 in worth and the vehicle is only worth $5000, you are paying for insurance that exceeds what you could get back.
Being over-insured means that you are carrying more replacement value or actual cash value insurance than your property is worth. For example, carrying a $20,000 policy on a $14,000 diamond ring. Most insurers underwrite to value and being over-insured is not a common practice or good business sense for insurers.