Update: songbird solved the problem:). I downloaded the exorcist add-on and adapted the clones.js file with this:
var str = item.getProperty(SBProperties.artistName) + '|' +
//item.getProperty(SBProperties.albumName) + '|' +
item.getProperty(SBProperties.trackName);// + '|' +
//item.getProperty(SBProperties.duration);
//item.getProperty(SBProperties.genre) + '|' +
//item.getProperty(SBProperties.contentLength) + '|' +
It then shows me a list of all files with identical artist and title. This gives me a total of 800 files, thus 400 duplicates. By default the line item.getProperty(SBProperties.albumName) was uncommented and then only 26 files were given!!