Yes you can double your rent if you furnish your property - to learn more about managing the property and to find the right tenants go to Corporate Housing by Owner and also take a look at the blog for furnishing tips

This is more of a benefit to you unless you are catering to a student or someone moving here for only a short period. So i would say not much but maybe offering it as an option, but not an increase!

Furnishings typically represent a value of about 15% the value of the property. Furnishing a rental would typically be a feature for a specific prospective tenant's interest. You should look at who the potential tenant is that is renting in your properties area, what do they do for a living, what do they do for enjoyment, what do they like for sports, outdoor activities, commuting, dining, cultural enrichment, charities etc. Is the target tenant in your area interested in a pre-furnished apartment? You should consult with several local real estate agents/brokers and get their imput as to whether renting your property fully furnished would be attractive to prospective tenants. In some areas it might be, in others not so much. Terms & timeline of tenancy agreement will also affect the types of tenants you'll attract.

Hope that helps,