laravel数据迁移主键出了问题

public function up()
    {
        Schema::create('tests', function (Blueprint $table) {
            $table->string('id',18);
            $table->primary('id');
            $table->integer('a',10)->nullable()->comment("aaaaaa");
            $table->string('b',20)->nullable()->comment("bbbbbb");
            $table->integer('c')->default(0)->comment("aaaaa");
        });
    }

今天发现一个问题数据写入不了表，这张表是上次建的，结果我打开表发现结构不对。我又测试了一下，结果报错

[Illuminate\Database\QueryException]
  SQLSTATE[42000]: Syntax error or access violation: 1068 Multiple primary key defined (SQL: alter table `tests` add primary key `tests_id_primary`(`id`))


[PDOException]
  SQLSTATE[42000]: Syntax error or access violation: 1068 Multiple primary key defined

没看懂 为什么会报这个错。