Prefix page's slug like "/page/{slug}"

bea-prefix-pages-slug.php

<?php
/**
 * Change post type single slug
 */
add_filter( 'register_post_type_args', 'bea_edit_page', 20, 2 );
function bea_edit_page( $args, $post_type ) {
	if ( 'page' !== $post_type ) {
		return $args;
	}

	$args['rewrite']['slug'] = 'page';

	return $args;
}

/**
 * Change page's link
 */
add_filter( 'page_link', 'bea_page_rewrite', 20, 3 );
function bea_page_rewrite( $link, $post_id, $sample ) {
	if ( 'page' !== get_post_type( $post_id ) ) {
		return $link;
	}

	$title = sanitize_title( get_the_title( $post_id ) );

	return str_replace( $title, sprintf( 'page/%s', $title ), $link );
}

/**
 * Handle duplicate content
 */
add_action( 'template_redirect', 'bea_redirect' );
function bea_redirect() {
	if ( ! is_page() || ! strpos( '/page/', $_SERVER['REQUEST_URI'] ) ) {
		return;
	}

	wp_safe_redirect( get_page_link(), '301' );
	exit;
}