Does using modules Metatags and Metatags Quick together cause SEO problems? I am checking the SEO of my Drupal 7 site, where I installed the Metatag and the Metatag Quick modules.

Does using both the modules together on a site cause SEO problems?
You have to be careful so that the output of each module won't overlap. For instance, the same meta should not be present twice and even worse with even slightly different values. I am not aware of the exact SEO penalty depending on the engine, but it is common sense.

You can examine the output of the modules for the &lt;head&gt; tag with the browser's inspector, or Firebug.