Update event not triggered every save (called every 3 saves...)

Hello!

It's realy weird but, the update event is called every time I've saved my file 3 times.
I'm using gulp and here is my config:
```JS
var gulp = require("gulp");
var browserify = require('browserify');
var watchify = require('watchify');
var babelify = require('babelify');
var uglify = require("gulp-uglify");
var buffer = require("vinyl-buffer");
var source = require("vinyl-source-stream");
var gutil = require("gulp-util");
var sourcemaps = require("gulp-sourcemaps");
var errorify = require("errorify");

gulp.task("default", function () {
let bundler = browserify({
entries: "./src/js/app.js",
debug: true,
});

return bundle(bundler);
});

gulp.task("watch", function () {
let bundler = browserify({
cache: {}, packageCache: {},
entries: "./src/js/app.js",
debug: true,
}).plugin(watchify, errorify);

bundler.on('update', function () {
bundle(bundler);
});

bundler.on('log', function (msg) {
console.log(msg);
});

bundle(bundler);
});

function bundle(bundler) {
return bundler.transform(babelify)
.bundle()
.pipe(source("main.js"))
.pipe(buffer())
.pipe(sourcemaps.init({loadMaps: true}))
// .pipe(uglify())
// .on("error", gutil.log)
.pipe(sourcemaps.write("./"))
.pipe(gulp.dest("./dist/js"));
}
```

I'am using Node v10.13.0

And here are my devDependecies:
```JS
"devDependencies": {
"@babel/core": "^7.1.2",
"@babel/preset-env": "^7.1.0",
"babelify": "^10.0.0",
"browserify": "^16.2.3",
"errorify": "^0.3.1",
"gulp": "^3.9.1",
"gulp-sourcemaps": "^2.6.4",
"gulp-uglify": "^3.0.1",
"gulp-util": "latest",
"vinyl-buffer": "^1.0.1",
"vinyl-source-stream": "^2.0.0",
"watchify": "^3.11.0"
}
```

Thank you :)