in your theme is a file called functions.php
Open it in the online WP theme editor and paste in these lines of code BEFORE the last line (which is a closing PHP tag ?> )
make sure this code is before that tag, and make sure you add nothing, not even a single blank space, after the closing tag in the file.
function excerpt_ellipse($text) {
return str_replace('[...]', ' <a href="'.get_permalink().'">Read more...</a>', $text); }
add_filter('the_excerpt', 'excerpt_ellipse');
This code replaces the [...] at the end of the excerpt with a "read more" clickable link.
To deal with SEO concerns, install the All in One SEO Pack plugin, or a similar plugin. The main issue with SEO and WordPress is not to create duplicate content. The plugin, out of the box, keeps Google from indexing content as duplicate. The idea is its single post pages (and tag pages) that should be indexed, not category, archive pages etc. The plugin comes pre-configured that way.