There is a problem that affects a minority of users when installing an extension with this error. This is because of a MySQL bug – a quick search on Google shows the issue.
You will need a revised SQL file to resolve the issue. You are advised to disable the extension temporarily to prevent your site from experiencing any more issues in the mean time. If the the complaint is about the unique keys, go to your SQL install script in appcodelocalWebshopappsProductmatrixsqlproductmatrix_setup (using Product Matrix as example) and open the latest version into a text editor.
Find the line declaring UNIQUE KEY and remove this line. On the PRIMARY KEY line above, delete the last comma “,” and save. Upload this over existing install script, clear your cache and keep disabled. Refresh the front end – if no errors are picked up you are resolved.
Otherwise, Disable the extension by renaming the relevant extension file from app/etc/modules, to e.g. Webshopapps_ProductMatrix.xml.orig refresh front end, rename and remove .orig and refresh again.