title: Cannot remigrate migration after migration rollback Laravel 5.2
body: <p>Good day, I'm new to laravel I was doing a migration rollback and it was successfully done</p>
  
  <blockquote>
    <p>Rolled back: 2018_02_22_172102_adding_fk_constrains_products_to_product_types_and_service_sub_types_table</p>
  </blockquote>
  
  <p>But when I try to re-migrate I encountered this error below. BTW I don't want to drop the column because it already existed and I don't want to lose the existing data in that column. I only want to add constraint between those tables</p>
  
  <blockquote>
    <p>[Illuminate\Database\QueryException]
        SQLSTATE[23000]: Integrity constraint violation: 1022 Can't write; duplicate key in table '#sql-2fc8_17c' (SQL: alter table <code>products</code> add constraint <code>products_product_type_id_foreign</code>
        foreign key (<code>product_type_id</code>) references <code>product_types</code> (<code>id</code>) on update cascade)</p>
    
    <p>[PDOException]
        SQLSTATE[23000]: Integrity constraint violation: 1022 Can't write; duplicate key in table '#sql-2fc8_17c'</p>
  </blockquote>
  
  <pre><code>Table     Non_unique  Key_name                            Seq_in_index  Column_name
  --------  ----------  ----------------------------------  ------------  -------------------
  products           0  PRIMARY                                        1  id             
  products           1  products_product_type_id_index                 1  product_type_id  
  products           1  products_service_sub_type_id_index             1  service_sub_type_id
  </code></pre>
  
  <p>this my migration code</p>
  
  <pre><code>public function up()
  {
  
      Schema::table('products',function (Blueprint $table){
          $table-&gt;integer('product_type_id')-&gt;unsigned()-&gt;index()-&gt;change();
          $table-&gt;foreign('product_type_id')-&gt;references('id')-&gt;on('product_types')-&gt;onUpdate('cascade');
  
          $table-&gt;integer('service_sub_type_id')-&gt;nullable()-&gt;unsigned()-&gt;index()-&gt;change();
          $table-&gt;foreign('service_sub_type_id')-&gt;references('id')-&gt;on('service_sub_types')-&gt;onUpdate('cascade');
  
      });
  }
  
  /**
   * Reverse the migrations.
   *
   * @return void
   */
  public function down()
  {
  
      Schema::table('products', function(Blueprint $table){
          $table-&gt;dropForeign(['product_type_id']);
          $table-&gt;dropForeign(['service_sub_type_id']);
      });
  
  }
  </code></pre>