If you use the URL as the link text, it will be converted to a hyperlink in the wiki, causing a double link.
You must replace the text to fix it:
<p><a href="http://www.youtube.com/watch?v=W4uipnBkj-4">some texte</a></p>
Using Moodle 2.6.2
Similar answer to the one from Visvanath
Have you tried turning off the autolink filter for the wiki? Should be doable from Administration block -> Wiki administration -> Filters
Ensure you turn off the filter "Convert URLs into links and images".
Otherwise, when editing the wiki page with the TinyMCE editor on, select the link and click the "No auto link" icon.
Hope this helps!
Thanks a lot for your replies. Sorry to get back so late. I just had a baby.
I cannot see how to disable wiki autolinking in Moodle 2.6. There is no wiki filter and no global settings for wiki activity.
I got around this by entering something other than the URL as the text of the link.
In html, your href should look something like this (with "video" being variable, as long as it isn't the url of the youtube video):
Not like this: