The double suffix issue is because you have probably migrated from M1 to M2.
You can solve it in .htaccess like this:
RewriteEngine On
RewriteBase /
# START exact file names to exclude
RewriteRule ^(a.html) - [L]
# END exact file names to exclude
RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s([^.]+)\.html [NC]
RewriteRule ^ %1 [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.html -f
RewriteRule ^(.*?)/?$ $1.html [NC,L]