If the Insured has other insurance against the loss covered by this policy, this policy shall be excess over any other valid and collectible insurance and shall then apply only in the amount by which the applicable limit of liability of this policy exceeds the sum of the applicable limit of liability of all such other insurance. This provision will not apply if the Insured has similar coverage with the same carrier or one of its affiliates. In this event only one limit of liability shall be available to the Insured, and such limit shall be the greater of the available limits irrespective of under which policy such limit is provided.