This number (1424837531
) is added by Sitefinity to ensure the uniqueness of the URLs. Seems you already have an item generated with the same URL.
However the scope of this check is not only the item provider, it checks all items URLs in all generic content based providers. So, the item with the same path could exists in any of the generic content providers.
We know that this is not a good solution and hope to find a better one in the future.
We are sorry for the inconvenience.
the Telerik team