Duplicate Oracle Sequence

I have a problem about using oracle sequence with yajra/laravel-oci8 library (v.5.1) in Laravel 5.1.

My scenario are:

1. I have a column named trailer_id in table A.

2. That column contain ID that generated using sequence named trailed_id_seq. Code:

   function postCreate(Request $request) {
       $id = $request->input('id');
   
       DB::beginTransaction();
   
       ....
   
       $running_no_seq = DB::getSequence();
       $trailer_id = $running_no_seq->nextValue('trailer_id_seq')."";
   
       for($i=0; $i < $counter; $i++) {
          // Some process, it can take a while here...
       }
   
       DB::table('A')->where('id',$id)->update([
          'trailer_id' => $trailer_id
       ]);
   
       DB::commit();
   }
   
   

   The property of trailer_id_seq sequence (captured in Navicat apps):

3. This function can be called by many users.

The problem is:

Why the trailer_id column can be duplicate when accessed by different user in different time?

Here some example: (created_by is user ID)

My question: How is this possible? How can i fixed it? (I can't make trailer_id column to unique)

I've read yajra oci docs: https://yajrabox.com/docs/laravel-oci8/master/sequence , is the code DB::getSequence()->nextValue('seq_name'); ONLY return next value and not updated the sequence?

Because in oracle docs, nextval is increments the sequence and returns the next value (https://docs.oracle.com/cd/A84870_01/doc/server.816/a76989/ch26.htm#4062).

Please help me. Thank you for any answer!

via Chebli Mohamed