You will most likely have some links are pointing to the same page, but the URL may be different. You need to tell the search engine which one is the real one: that's the canonical URL.
For instance, you may have
You need to set the header Link
header("Link: <https://example.com/index.php>; rel=\"canonical\"");
And/Or, you need to set the meta tag
<link rel="canonical" href="https://example.com/index.php" />