This Grunt config will only run a task on the file that has triggered the watch plugin.

Gruntfile.js

/*
  automatically sync files from the deploy directory back to the source directory
*/
module.exports = function( grunt ){

	grunt.initConfig( {
		copy : {
			main: {
				files : [
				]
			}
		},
		watch : {
			js : {
				files : [
					getDirForType( 'js', 'web' )
				],
				tasks : [
					'copy'
				],
				options : {
					spawn : false
				}
			}
		}
	} );

	grunt.event.on( 'watch', function( action, filepath, taskName ){
		
		var files = grunt.config( 'copy.main.files' ),
			fileName = new RegExp('/' + taskName + '.+').exec( filepath )[0];
		
		files.push(
			{
				src : filepath,
				dest : deployDir + fileName
			}
		);
		
		grunt.config( 'copy.main.files', files );

	} );

	require( 'matchdep' ).filterDev( 'grunt-*' ).forEach( grunt.loadNpmTasks );
};