Title tag in header.php creates a 2nd title tag. Please advise.

I am studing wp seo and am watching this YouTube video https://youtu.be/Cj6jF5p4Va8?t=2m37s created for this blog post: https://teslathemes.com/blog/how-to-optimize-your-wordpress-site-without-any-seo-plugins/

Step 3 : Adding title and meta descriptions

… which tells me to add this title tag code to the header.php for my theme (Divi):

<title><?php bloginfo(‘name’); ?><?php wp_title(); ?></title>

Now I have a title tag twice. See source code – the shorter one is the result of following step 3.

WC3 html checker validator (validator.org) says I should not have 2 title tags.

If I see in source code that I already have a title tag, yet its not in the header.php and when added to the header.php I now see title tag twice in source code (the later one is shorter), do I leave it or remove it from header.php? Is it bad for SEO to have 2 title tags? Or remove it from the first source and if so how to find where that is done?

Do I need to create a child theme if I want to keep the header.php title tag to avoid losing it on updates?

Reminder: I am doing this for SEO and new to wp seo. Please advise.

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