There are two issues.
1. There is some bug/feature in blends-dev: You need to insert
an empty line at the end of your debian/control.stub file
(probably I should add such a line in any case to fix this).
Adding this will solve the duplicate field Package issue.
However
2. You probably do not want the completely empty openstudio
package which will be created by the last two lines in your
control.stub file which was missleading in the outdated
examples (just updated it in SVN).
This also fixes the problem (if you leave the empty line
in the end).