Please, don't recommend me the long and very detailed thread with more than 173 upvotes. It didn't work for me. I have also tried many others (1, 2, 3, 4). They all give me TOO_MANY_REDIRECTS or error 500. So, here is my issue:

With my current .htaccess, this is what happens:

https://www.dukescasino.com/ - works perfectly

https://dukescasino.com/ - redirects to the above which is great

The two options below loads fine, but it should be redirecting to the https version:

http://www.dukescasino.com/

http://dukescasino.com/

Here is the current .htaccess:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /

# BEGIN WordPress
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

I don't believe it is relevant, but if so, here is the list of current active plugins:

• Advanced Custom Fields
• All In One SEO Pack
• Bop Search Box Item Type For Nav Menus
• Contact Form 7
• Disable Comments
• Google XML Sitemaps
• Jetpack by WordPress.com
• Search & Filter
• Slider WD
• TablePress
• UpdraftPlus - Backup/Restore
• Wordfence Security
• WPide
• WP Smush
• WP Super Cache

Edit 1 - Tests performed:

Test A:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /

RewriteCond %{HTTPS} off
# First rewrite to HTTPS:
# Don't put www. here. If it is already there it will be included, if not
# the subsequent rule will catch it.
RewriteRule .* https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
# Now, rewrite any request to the wrong domain to use www.
RewriteCond %{HTTP_HOST} !^www.
RewriteRule .* https://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

# BEGIN WordPress
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Result: ERR_TOO_MANY_REDIRECTS

Test B:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /

RewriteCond %{HTTPS} !=on
RewriteRule ^.*$ https://%{SERVER_NAME}%{REQUEST_URI} [R,L]

# BEGIN WordPress
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Result: ERR_TOO_MANY_REDIRECTS

Test C:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /

RewriteCond %{SERVER_PORT} ^80$
 RewriteRule ^.*$ https://%{SERVER_NAME}%{REQUEST_URI} [R=301,L]

# BEGIN WordPress
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Result: ERR_TOO_MANY_REDIRECTS

Test D:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /

RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

# BEGIN WordPress
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Result: ERR_TOO_MANY_REDIRECTS

Test E:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /

RewriteCond %{HTTP_HOST} !^www.
RewriteRule ^(.*)$ https://www.%{HTTP_HOST}%{REQUEST_URI}$1 [R=301,L]
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI}$1

# BEGIN WordPress
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Result: 302 found. Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.