Duplicate Category and Page URL loads Category instead of the Page

Load Page when a Page and a Category archive have the same URL:

This is default WordPress behaviour: When you have the same URL for a category archive & for a page, WordPress will load the page instead of the category archive.

So unless you have a plugin that is changing this behavior for your WordPress setup, your duplicate URL should load the page, not the category archive.

To achieve the proper URL Structure:

The URL structure you wanted needs some work. So unless you’ve done it already, follow the instructions below to achieve the said URL structure:

# URL structure for Pages:
https://example.com/page-slug

This happens by default, you don’t need to do anything.

---

# URL structure for Categories
https://example.com/category-slug

For this:

1. Go to: WordPress Admin Panel Menu → Settings.
2. Put a single dot (.) in the Category base text field.
3. Click Save Changes button.

---

# URL structure for Posts
https://example.com/category-slug/post-slug

For this:

1. Go to: WordPress Admin Panel Menu → Settings.
2. Select Custom Structure and enter /%category%/%postname%/ in the Custom Structure text field.
3. Click Save Changes button.

If for some reason you are not getting the expected result from this, then use this URL structure in a WordPress installation with:

1. WordPress Core updated to the latest version.
2. No plugin is activated
3. A default theme like Twentyseventeen is activated

and then see what happens with duplicate Page and Category URL.

Solving the Category Pagination Problem:

If you check this post you’ll see that this kind of URL structure will cause some pagination problem in your category archive pages. This happens because WordPress confuses with /page/2 part as a different page or post.

For example: say you have a category named service and service category has posts like web development, hosting etc. with the following URL:

https://example.com/service
https://example.com/service/web-development
https://example.com/service/hosting

Now you may have multiple pages in your service category argive page, like:

https://example.com/service
https://example.com/service/page/2
https://example.com/service/page/3

Because of this URL structure, WordPress thinks you are trying to load a post that has the slug post under the service category and that post is a paginated post.

So instead of loading the next pages of your service category archive, WordPress will try to load a paginated post with the slug page and you’ll get a 404 (page not found) error instead.

To fix this, you may use a simple plugin with the following CODE:

<?php
/*
Plugin Name:  Category Pagination Fix
Plugin URI:   https://wordpress.stackexchange.com/a/308826/110572
Description:  Fix category pagination for possible conflicts with page or post url
Version:      1.0.0
Author:       Fayaz Ahmed
Author URI:   https://www.fayazmiraz.com/
*/

function wpse308326_fix_category_pagination( $query_string = array() )
{
    if( isset( $query_string['category_name'] )
            && isset( $query_string['name'] ) && $query_string['name'] == 'page'
            && isset( $query_string['page'] ) ) {
        $paged = trim( $query_string['page'], "https://wordpress.stackexchange.com/" );
        if( is_numeric( $paged ) ) {
            // we are not allowing 'page' as a page or post slug 
            unset( $query_string['name'] );
            unset( $query_string['page'] )  ;

            // for a category archive, proper pagination query string  is 'paged'
            $query_string['paged'] = ( int ) $paged;
        }
    }   
    return $query_string;
}
add_filter( 'request', 'wpse308326_fix_category_pagination' );