Unexpected Error: write callback called multiple times executing Gulp.js tasks inside a loop - gulp

Question

I'm trying to execute some gulp tasks inside a map loop to generate css files for each sass theme I defined.
Finally, I return all merged tasks with merge-stream package.
Unfortunately, callback seems sent for each iteration / gulp task of my loop and i'm getting the following message :
Error: write callback called multiple times
#Task('build')
build() {
var self = this;
var tasks = this.themes.map((theme) => {
this.foldersXml = getAttributes('jntFolderWithExternalProvider');
this.filesXml = getAttributes('jntFolder');
this.foldersXml[theme] = []; this.foldersXml[theme].push(getAttributes('jntFolderWithExternalProvider'));
this.filesXml[theme] = []; this.filesXml[theme].push(getAttributes('jntFolderWithExternalProvider'));
fs.readFile(this.themesFolder + '/' + theme + '/' + this.fileName, 'utf8', (err: Error, data: string & Buffer) => {
if (err) {
throw new gutil.PluginError({
plugin: 'build',
message: 'Main file doesn\'t exist for the theme: ' + theme
});
} else {
var vars = data.match(/\$(.*?)\:/g);
this.requiredVars.map(requiredVar => {
if(vars !== null){
if(!vars.contains(requiredVar)){
throw new gutil.PluginError({
plugin: 'build',
message: 'Required variable ' + requiredVar + ' is not defined'
});
};
}
});
}
});
return gulp.src(this.templatesFolder + '/' + this.pattern)
.pipe(header('#import \'' + this.themesFolder + '/' + theme + '/' + this.fileName + '\';'))
.pipe(sass.sync().on('error', gutil.log))
.pipe(rename(function (path: any) {
var file = path.basename + path.extname;
var folderXml = getAttributes('jntFolderWithExternalProvider') as any;
folderXml[file] = [ getAttributes('jntCssFile') ];
self.foldersXml[theme][0][file] = []; self.foldersXml[theme][0][file].push(folderXml);
var fileXml = getAttributes('jntFolderWithExternalProvider') as any;
fileXml['jcr:content'] = [ getAttributes('jntRessource') ];
self.filesXml[theme][0][file] = []; self.filesXml[theme][0][file].push(fileXml);
path.dirname += '/' + file;
}))
.pipe(gulp.dest(this.themesFolder + '/' + theme))
});
return merge(tasks);
}
#SequenceTask()
run(){
return ['build'];
}
I just want to get rid of this message and be sure that callback is invoked only at the end. What's the best approach for you ?