If the tools are for both developers and business users, how is duplication of applications reduced (since we know that the communication between the two communities isn't necessarily 'open')?

The mashup software must promote sharing and re-use.  This is what makes a mashup platform produce real value to an organization.  We’ve written a bit on this in http://blogs.jackbe.com/2008/10/mashup-recycling-now-this-is-green-it.html and http://blogs.jackbe.com/2009/02/it-pays-to-be-virtuous.html.