but my current leader has two. It's very curious. I picked a ready-made/ preset player avatar and whenever I do diplomacy I now notice I have two strengths. Is this intended? If so, how do I change my strengths?

I noticed it too, those were not the traits I chose.

Did it does an effect ?

I was curious about this too. My avatar has "Ideologist +5 inf on main plaza" and "Artist 1star + 2 inf on commons quarter". I cannot see the actual effect in game on my main plaza. Shame as i think this would be a good addition to the game.

I played a wee bit more of the game and checked for a bonus. I found nothing. Here is a screenshot of the capital city, everything seems normal: