Laravel 5: Integrity constraint violation: 1062 - Many to Many

I need some advice on the following.

I have 2 migrations, like so.

        Schema::create('plates', function (Blueprint $table) {
        $table->increments('id');
        $table->integer('serial_number');
        $table->string('crc-code', 50);
        $table->string('reason', 50)->nullable();

        $table->softDeletes();

        $table->timestamps();
    });

And another one

  Schema::create('documents', function (Blueprint $table) {
        $table->increments('id');
        $table->string('name', 25)->unique();
        $table->text('description')->nullable();
        $table->longText('relative_path');

A pivot table for many to many relations I have setup just like this

        Schema::create('document_plate', function (Blueprint $table) {
      $table->integer('plate_id')->unsigned()->index();
      $table->integer('document_id')->unsigned()->index();

      $table->primary(['plate_id', 'document_id']);

      $table->timestamps();
    });

When taking a certain action I use the code below to attach a plate to a document

       $plate  =   Plate::find(1);
       $doc    =   Document::find(1);
       if($plate && $doc) {
          $plate->documents()->attach($doc->id);
       }

The first time, everything works just fine! the document_plate gets updated. The error occurs when having the same ids doing it again.

SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '1-1' for key 'PRIMARY' (SQL: insert into document_plate (created_at, document_id, plate_id, updated_at)

Now the question

Is there a way to avoid getting errors and just update the table with the same ids??

Or.. I need to setup a some sort of validation on the front-end which tells the user (before submitting) that he/she choose the same id's which are already in the table.

Note: I am using AngularJS for front-end operations.