Grunt will clean up your build with the grunt-contrib-clean to make sure that no artifacts from previous builds are hanging around.
I noticed that you used concat before using uglify. Uglify has the ability to concatenate files, so concatenating before uglifying is not necessary.
See this from the grunt-contrib-uglify documentation: https://www.npmjs.com/package/grunt-contrib-uglify#basic-compression
Notice that more than one source file is being directed to a destination file in their first example; all of these get concatenated before being uglified.