Error occurred in deployment step 'Add Solution': A feature with ID {Guid} has already been installed in this farm. Use the force attribute to explicitly re-install the feature.

The fix in Visual Studio 2010 is to double click on the feature and open the feature window. from there use the property window and set the value AlwaysForceInstall="TRUE".

<Feature AlwaysForceInstall="TRUE">
....
</Feature>

Also, you can use power-shell:

Install-SPSolution -Identity <Solution File> -GACDeployment -Force