This is a known bug in Joomla for the last several years. I proposed a change a few months ago where the canonical tag is only included if the current page is not the canonical URL, which is how it is supposed to be according to Google. That change was accepted. Without that change, you would have the same situation as now, just that the "right" page would have the canonical tag, too. Which again is wrong by Googles own definition of the canonical tag.

If you want to solve this issue, help by supporting my routing related PRs. They are moving towards that and try to solve the issue.

I'd love to support..... I am not that familiar with how it works here (I am a github.com noob) tell me which ones and how to promote.. Tx!

Can someone explain why the plugin code for rel canonical is comparing an absolute URL with a relative ?

$router = $app::getRouter();

$uri     = JUri::getInstance();
$domain  = $this->params->get('domain');

if ($domain === null || $domain === '')
{
    $domain = $uri->toString(array('scheme', 'host', 'port'));
}

$link = $domain . JRoute::_('index.php?' . http_build_query($router->getVars()), false);

if ($uri->toString() !== $link)
{
    $doc->addHeadLink(htmlspecialchars($link), 'canonical');
}

• if you save SEF plugin once, you get into the parameters: domain:false

thus
if ($domain === null || $domain === '') {...}
is not executed and this
if ($uri->toString() !== $link)
is comparing an absolute URL with a relative
is this done deliberately ? is this what we want the plugin to do ?

can someone explain what is the intention of code:
if ($domain === null || $domain === '') {...}
why is not ?:
if ($domain == '') {...}

I can confirm that this bug still exist on Joomla 3.4.8.

We have modified the code in 3.5.0

I have still issues with the sef plugin, also in 3.5.0 . When I add $domain === null ($domain is null when I never saved the plugin config) the comparision is done between absolute urls and fixes the problem:

if ($domain === null || $domain === false || $domain === '') {...}

@elpescador-nl Are you suggesting a fix?

@roland-d

That code J3.5.0 code, no longer exists in J3.5.1+, comment is no longer valid,

the issue is fixed in J3.5.1 ,

• the SEF plugin can never guess what the canonical URL should be, so no canonical is added at all, so it is now the current URL, unless component decides otherwise
• the SEF plugin will only care to fix the alternative domain ... please read the initial posting of this PR, and all of the postings to know more:

#9565

I think this can be closed

@ggppdk Thank you for your comment. Closing this now, it can always be reopened if needed.