php twig增量变量和连接表行

Question

in my mysql db I got the following data:

 row   value 
--------------
|row1|   12  |
--------------
|row1|   13  |
--------------
|row2|   15  |
--------------

which I transform to the following php array($myarray):

[arr]=>
  array(2) {
    ["row1"]=>
    array(2) {
      [0]=>
      int(12)
      [1]=>
      int(13)
    }
    ["row2"]=>
    array(1) {
      [0]=>
      int(15)
    }
  }

which I display with the following code in twig:

{% for key, value in myarray %}
  <tr>
     <td>{{key}}</td>
     <td>{{value|join(', ')}}</td> 
  </tr>
{% endfor %}

so it looks like the following:

---------------
|row1| 12, 13 |
---------------
|row2| 15     |
---------------

My problem is that when the values for row1 and row2 are equal I need to show them in one table row, like the following (say that values for row2 are also 12 and 13 instead of 15):

---------------------
|row1, row2| 12, 13 |
---------------------

I tried to do that with twig variables like this:

{% set inc = 0 %}
  {% for key, value in myarray %}
    {% set foo ~ inc = [key, value] %}
    {% set inc = inc + 1 %}
  ...table html here 
  {% endfor %}

but still cannot join the rows in case arrays for 'row1' and 'row2' are equal. Any ideas how to do that? (There may be more than 2 'row1' and 'row2' in resulting array). Thank you.