I can't be 100 % sure that this is caused by Artio Joomsef, but I have looked at many Joomla sites and many sites that use Artio Joomsef has the same "problem" as mine.

"Problem" on quotes, because it's not a big problem, it's just an annoyance. This image explains what I mean:



I want the site name to appear only once. Not twice.

Can someone tell me whether or not Artio Joomsef is the culprit or not? Any idea about how I can solve it?

Check the plugin System - ARTIO JoomSEF

i had same issue and i remove title generation from template index.php and config the plugin .
you can add with this before after or not add at all the title.