The purchase of rental car insurance coverage from the rental company is optional;
whether or not you have any coverage of your own or not.
See our Car Rental Insurance Overview
The company expect that you be aware that if you decline the optional loss damage waiver or collision damage waiver offered at the rental counter, that you are fully financially responsible for any loss of (ie theft) or damage to the rental vehicle while it's under contract to you. This will often include administrative fees and/or loss of use fees should anything happen to the rental car.
Whether or not you feel the need to purchase car rental insurance from the rental company, will probably depend on just how covered you are through your own personal automobile insurance with Geico.
The comprehensive collision insurance that you have with Geico may fully cover you for loss, damage or theft of a rental car.... or it may not. You should contact a Geico representative and have them clarify for you exactly to what extent you will be covered for the rental. It's best, in your situation, to determine is rental car insurance necessary or does Geico provide the coverage you would want?
Read more about Personal Auto Insurance Coverage
You should contact your Geico representative and determine exactly to what extent you are covered for in a car that you rent. You will want to make note of any exceptions to coverage or any terms and conditions of coverage (ie, type of rental car, length of rental).
When trying to determine is rental car insurance necessary, you will want to find out from Geico:
Is coverage up to the full value of the rental car, or only up to the value of your own personal car?
Are you subject to the same deductible, should something happen?
Are there any restrictions geographically (certain cities, states, countries) on where you are covered (or not covered) while in the vehicle?
Is the Geico coverage adequate for damage (if you do have it) and does it apply only when you are driving the vehicle, or would it apply if something happened while another authorized driver was driving. Determine if rental car insurance is necessary from the rental car provider in order to cover any other drivers?
Does coverage offered by Geico cover theft of the vehicle, loss of use charges and the administrative fees charged by the rental car company?
Hope this helps!