You are trying to create a redirect, but the following {{plural|{{{1}}}|page is already a redirect|{{{1}}} pages are already redirects}} to this page.  Clicking 'save' will therefore create {{plural|{{{1}}}|a double redirect from this page|double redirects from these pages}}:

<span class="list of double redirects to be created by this edit">
# [https://sitename.com/wiki/{{{2}}}&redirect=no]
# [https://sitename.com/wiki/{{{3}}}&redirect=no]
</span>

use MediaWiki\EditPage\Constraint\IncomingRedirectDoubleRedirectConstraint;

<?php
/**
 * Prevent creating double redirects by checking incoming redirects to the page.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 */

namespace MediaWiki\EditPage\Constraint;

use MediaWiki\Content\Content;
use MediaWiki\Linker\LinkTarget;
use MediaWiki\Title\Title;
use MediaWiki\Status\StatusValue;
use Wikimedia\Rdbms\IDatabase;

class IncomingRedirectDoubleRedirectConstraint implements IEditConstraint
{
    private Content $newContent;
    private LinkTarget $title;
    private IDatabase $db;
    private string $result = '';
    /** @var Title[] List of incoming redirect titles */
    private array $incomingRedirects = [];
    private ?LinkTarget $newRedirectTarget = null;

    /**
     * @param Content $newContent The new content of the page being edited
     * @param LinkTarget $title The title of the page being edited
     * @param IDatabase $db Database connection for querying redirects
     */
    public function __construct(Content $newContent, LinkTarget $title, IDatabase $db)
    {
        $this->newContent = $newContent;
        $this->title = $title;
        $this->db = $db;
        $this->newRedirectTarget = $newContent->getRedirectTarget();
    }

    /**
     * Check if the new edit would create a double redirect by looking for incoming redirects.
     *
     * @return string One of self::CONSTRAINT_PASSED or self::CONSTRAINT_FAILED
     */
    public function checkConstraint(): string
    {
        // Only check if the new content is a redirect
        if ($this->newRedirectTarget === null) {
            $this->result = self::CONSTRAINT_PASSED;
            return self::CONSTRAINT_PASSED;
        }

        // Query for pages that redirect to this page (incoming redirects)
        $conds = [
            'rd_namespace' => $this->title->getNamespace(),
            'rd_title' => $this->title->getDBkey(),
            'rd_interwiki' => '',
        ];

        $res = $this->db->newSelectQueryBuilder()
            ->select(['rd_from'])
            ->from('redirect')
            ->where($conds)
            ->caller(__METHOD__)
            ->fetchResultSet();

        $incomingRedirects = [];
        foreach ($res as $row) {
            // Convert page ID (rd_from) to Title object
            $redirectingTitle = Title::newFromID($row->rd_from);
            if ($redirectingTitle !== null) {
                $incomingRedirects[] = $redirectingTitle;
            }
        }

        if (count($incomingRedirects) > 0) {
            $this->incomingRedirects = $incomingRedirects;
            $this->result = self::CONSTRAINT_FAILED;
            return self::CONSTRAINT_FAILED;
        }

        $this->result = self::CONSTRAINT_PASSED;
        return self::CONSTRAINT_PASSED;
    }

    /**
     * Return a StatusValue describing the constraint result for legacy error handling.
     *
     * @return StatusValue
     */
    public function getLegacyStatus(): StatusValue
    {
        if ($this->result === self::CONSTRAINT_FAILED) {
            $status = StatusValue::newFatal('edit-constraint-incoming-double-redirect');

            // Format the list of incoming redirects as a comma-separated string
            $titlesText = array_map(
                fn(Title $title) => $title->getPrefixedText(),
                $this->incomingRedirects
            );

            $listText = implode(', ', $titlesText);

            // Add the list to the status details for the message
            $status->params = [$listText];

            return $status;
        }
        return StatusValue::newGood();
    }
}