P2 update failing because components already installed

I've just come across another p2 install problem.  It's happened a few times before - let's get the solution down.

Background
The scenario is that the p2 build process is working, and the repository is unpacking successfully.  A Debrief NG instance is open, and check for updates returns "There is nothing to update".

Problem
Whilst the repository contains updated features, the product is of the same version number.

Solution
Update the product version number in the debrief.product file.