gulp-dedupe 
Check for duplicates in the stream and filter them or throw an error.
Install:
npm install gulp-dedupe --save-dev
Example:
var dedupe = require('gulp-dedupe');
...
.pipe(dedupe())
.pipe(concat('bundle.css'))
.pipe(gulp.dest('./build'));
dedupe(options)
optionally accepts options
object. There are two options:
error
to emit an error in case of duplicate (false
by default).same
to emit an error in case duplicates have different contents (true
by
default).diff
to supply duplicates with different contents error (same: true
)
with actual diff (false
by default).