Users: Treat email addresses that differ only in letter case as the s…

src/wp-admin/css/list-tables.css

@@ -870,6 +870,18 @@ p.pagenav {
 	margin-top: 1px;
 }
 
+.column-email .duplicate-email {
+	margin: 0.3em 0 0;
+	color: #b32d2e;
+}
+
+.column-email .duplicate-email .dashicons {
+	font-size: 16px;
+	width: 16px;
+	height: 16px;
+	vertical-align: text-bottom;
+}
+
 .row-actions {
 	color: #646970;
 	font-size: 13px;

src/wp-admin/includes/class-wp-users-list-table.php

@@ -32,6 +32,15 @@ class WP_Users_List_Table extends WP_List_Table {
 	 */
 	public $is_site_users;
 
+	/**
+	 * IDs of users on the current page whose email address is also used by another
+	 * user, ignoring letter case. Keys are user IDs.
+	 *
+	 * @since 7.2.0
+	 * @var true[]
+	 */
+	protected $duplicate_email_user_ids = array();
+
 	/**
 	 * Constructor.
 	 *
@@ -411,11 +420,77 @@ public function display_rows() {
 			$post_counts = count_many_users_posts( array_keys( $this->items ) );
 		}
 
+		$this->duplicate_email_user_ids = $this->get_duplicate_email_user_ids( array_keys( $this->items ) );
+
 		foreach ( $this->items as $userid => $user_object ) {
 			echo "\n\t" . $this->single_row( $user_object, '', '', isset( $post_counts ) ? $post_counts[ $userid ] : 0 );
 		}
 	}
 
+	/**
+	 * Finds which of the given users share an email address with another user,
+	 * ignoring letter case.
+	 *
+	 * Many mailbox providers treat `abc@example.com` and `ABc@example.com` as the
+	 * same mailbox, so such accounts are likely duplicates.
+	 *
+	 * @since 7.2.0
+	 *
+	 * @global wpdb $wpdb WordPress database abstraction object.
+	 *
+	 * @param int[] $user_ids IDs of the users to check.
+	 * @return true[] Array keyed by the IDs of users whose email address is shared.
+	 */
+	protected function get_duplicate_email_user_ids( $user_ids ) {
+		global $wpdb;
+
+		$user_ids = array_filter( array_map( 'intval', $user_ids ) );
+		if ( empty( $user_ids ) ) {
+			return array();
+		}
+
+		$emails = array();
+		foreach ( $user_ids as $user_id ) {
+			$user = get_userdata( $user_id );
+			if ( $user && '' !== $user->user_email ) {
+				$emails[ strtolower( $user->user_email ) ] = true;
+			}
+		}
+
+		if ( empty( $emails ) ) {
+			return array();
+		}
+
+		$emails       = array_keys( $emails );
+		$placeholders = implode( ',', array_fill( 0, count( $emails ), '%s' ) );
+
+		/*
+		 * All users whose email matches, ignoring case, the email of a user on this page.
+		 * With the default case-insensitive collation a plain comparison already ignores
+		 * letter case and can use the user_email index; LOWER() is only needed otherwise.
+		 */
+		$email_column = _wp_is_user_email_case_sensitive() ? 'LOWER(user_email)' : 'user_email';
+
+		$matches = $wpdb->get_results(
+			// phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared, WordPress.DB.PreparedSQLPlaceholders.UnfinishedPrepare
+			$wpdb->prepare( "SELECT ID, user_email FROM $wpdb->users WHERE $email_column IN ($placeholders)", $emails )
+		);
+
+		$users_by_email = array();
+		foreach ( $matches as $match ) {
+			$users_by_email[ strtolower( $match->user_email ) ][] = (int) $match->ID;
+		}
+
+		$duplicates = array();
+		foreach ( $users_by_email as $ids ) {
+			if ( count( $ids ) > 1 ) {
+				$duplicates += array_fill_keys( $ids, true );
+			}
+		}
+
+		return array_intersect_key( $duplicates, array_flip( $user_ids ) );
+	}
+
 	/**
 	 * Generates HTML for a single row on the users.php admin panel.
 	 *
@@ -599,6 +674,12 @@ public function single_row( $user_object, $style = '', $role = '', $numposts = 0
 						break;
 					case 'email':
 						$row .= "<a href='" . esc_url( "mailto:$email" ) . "'>$email</a>";
+						if ( isset( $this->duplicate_email_user_ids[ $user_object->ID ] ) ) {
+							$row .= sprintf(
+								'<p class="duplicate-email"><span class="dashicons dashicons-warning" aria-hidden="true"></span> %s</p>',
+								__( 'Another user has this email address, possibly with different letter case.' )
+							);
+						}
 						break;
 					case 'role':
 						$row .= esc_html( $roles_list );

src/wp-includes/user.php

@@ -2134,16 +2134,42 @@ function username_exists( $username ) {
  * Conditional Tags} article in the Theme Developer Handbook.
  *
  * @since 2.1.0
+ * @since 7.2.0 The comparison is case-insensitive regardless of the database collation.
+ *
+ * @global wpdb $wpdb WordPress database abstraction object.
  *
  * @param string $email The email to check for existence.
  * @return int|false The user ID on success, false on failure.
  */
 function email_exists( $email ) {
+	global $wpdb;
+
 	$user = get_user_by( 'email', $email );
 	if ( $user ) {
 		$user_id = $user->ID;
 	} else {
 		$user_id = false;
+
+		/*
+		 * Most mailbox providers treat the local part of an address as case-insensitive.
+		 * The default users table collation already compares emails that way, so the
+		 * lookup above has covered it. Only when the column compares case-sensitively
+		 * (or the collation is unknown) fall back to an explicit case-insensitive lookup.
+		 * That lookup cannot use the user_email index, so it is avoided where possible.
+		 */
+		$trimmed_email = trim( (string) $email );
+		if ( '' !== $trimmed_email && _wp_is_user_email_case_sensitive() ) {
+			$found_id = $wpdb->get_var(
+				$wpdb->prepare(
+					"SELECT ID FROM $wpdb->users WHERE LOWER(user_email) = LOWER(%s) LIMIT 1",
+					$trimmed_email
+				)
+			);
+
+			if ( $found_id ) {
+				$user_id = (int) $found_id;
+			}
+		}
 	}
 
 	/**
@@ -2158,6 +2184,56 @@ function email_exists( $email ) {
 	return apply_filters( 'email_exists', $user_id, $email );
 }
 
+/**
+ * Determines whether the database compares user email addresses case-sensitively.
+ *
+ * The default collation of the users table is case-insensitive, in which case
+ * a plain comparison already matches email addresses that differ only in letter
+ * case, and can use the user_email index. Case-sensitive collations such as
+ * `*_bin` or `*_cs` need an explicit LOWER() comparison instead.
+ *
+ * When the collation cannot be determined, for example on a non-MySQL database,
+ * the comparison is assumed to be case-sensitive.
+ *
+ * @since 7.2.0
+ * @access private
+ *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
+ * @return bool True if user email comparisons are case-sensitive, false otherwise.
+ */
+function _wp_is_user_email_case_sensitive() {
+	global $wpdb;
+
+	static $is_case_sensitive = array();
+
+	if ( ! isset( $is_case_sensitive[ $wpdb->users ] ) ) {
+		$result = true;
+
+		if ( $wpdb->is_mysql ) {
+			$column = $wpdb->get_row( "SHOW FULL COLUMNS FROM $wpdb->users LIKE 'user_email'" );
+
+			if ( $column && ! empty( $column->Collation ) ) {
+				$result = ! str_ends_with( strtolower( $column->Collation ), '_ci' );
+			}
+		}
+
+		$is_case_sensitive[ $wpdb->users ] = $result;
+	}
+
+	/**
+	 * Filters whether user email comparisons are case-sensitive in the database.
+	 *
+	 * When true, email lookups that need to ignore letter case use LOWER(), which
+	 * cannot use the user_email index.
+	 *
+	 * @since 7.2.0
+	 *
+	 * @param bool $is_case_sensitive Whether the user_email column compares case-sensitively.
+	 */
+	return (bool) apply_filters( 'wp_is_user_email_case_sensitive', $is_case_sensitive[ $wpdb->users ] );
+}
+
 /**
  * Checks whether a username is valid.
  *

tests/phpunit/tests/admin/wpUsersListTable.php

@@ -30,4 +30,92 @@ public function test_get_views_should_return_views_by_default() {
 
 		$this->assertSame( $expected, $this->table->get_views() );
 	}
+
+	/**
+	 * @ticket 66238
+	 *
+	 * @covers WP_Users_List_Table::get_duplicate_email_user_ids
+	 *
+	 * @dataProvider data_email_case_sensitivity
+	 *
+	 * @param bool $is_case_sensitive Whether the database compares user emails case-sensitively.
+	 */
+	public function test_get_duplicate_email_user_ids_flags_case_variants( $is_case_sensitive ) {
+		global $wpdb;
+
+		$original = self::factory()->user->create( array( 'user_email' => 'abc@example.com' ) );
+		$variant  = self::factory()->user->create( array( 'user_email' => 'placeholder@example.com' ) );
+		$unique   = self::factory()->user->create( array( 'user_email' => 'unique@example.com' ) );
+
+		// Duplicates can no longer be created through the API, so simulate legacy data.
+		$wpdb->update( $wpdb->users, array( 'user_email' => 'ABc@example.com' ), array( 'ID' => $variant ) );
+		clean_user_cache( $variant );
+
+		add_filter( 'wp_is_user_email_case_sensitive', $is_case_sensitive ? '__return_true' : '__return_false' );
+
+		$method = new ReflectionMethod( $this->table, 'get_duplicate_email_user_ids' );
+		if ( PHP_VERSION_ID < 80100 ) {
+			$method->setAccessible( true );
+		}
+
+		$queries = array();
+		$collect = static function ( $query ) use ( &$queries ) {
+			$queries[] = $query;
+			return $query;
+		};
+		add_filter( 'query', $collect );
+
+		$flagged = $method->invoke( $this->table, array( $original, $unique ) );
+
+		remove_filter( 'query', $collect );
+
+		$this->assertSame( array( $original ), array_keys( $flagged ), 'Only the user sharing an address should be flagged, even when the other account is not on the page.' );
+
+		$lower_queries = preg_grep( '/LOWER\(user_email\)/', $queries );
+		if ( $is_case_sensitive ) {
+			$this->assertNotEmpty( $lower_queries, 'A case-sensitive collation should use a LOWER() comparison.' );
+		} else {
+			$this->assertEmpty( $lower_queries, 'A case-insensitive collation should not use LOWER(), so that the index can be used.' );
+		}
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_email_case_sensitivity() {
+		return array(
+			'case-insensitive collation' => array( false ),
+			'case-sensitive collation'   => array( true ),
+		);
+	}
+
+	/**
+	 * @ticket 66238
+	 *
+	 * @covers WP_Users_List_Table::display_rows
+	 */
+	public function test_display_rows_highlights_duplicate_email() {
+		global $wpdb;
+
+		$original = self::factory()->user->create( array( 'user_email' => 'abc@example.com' ) );
+		$variant  = self::factory()->user->create( array( 'user_email' => 'placeholder@example.com' ) );
+		$unique   = self::factory()->user->create( array( 'user_email' => 'unique@example.com' ) );
+
+		$wpdb->update( $wpdb->users, array( 'user_email' => 'ABc@example.com' ), array( 'ID' => $variant ) );
+		clean_user_cache( $variant );
+
+		$this->table->items = array(
+			$original => get_userdata( $original ),
+			$unique   => get_userdata( $unique ),
+		);
+
+		ob_start();
+		$this->table->display_rows();
+		$output = ob_get_clean();
+
+		$this->assertSame( 1, substr_count( $output, 'class="duplicate-email"' ), 'Exactly one row should be highlighted.' );
+		$this->assertMatchesRegularExpression( "#<tr id='user-{$original}'.*?class=\"duplicate-email\".*?</tr>#s", $output, 'The duplicate user row should be highlighted.' );
+	}
 }