I noticed this issue as I tend to view SO via the mobile site frequently:
The mobile versions of SO and meta (at least), don't show possible duplicate questions while you're typing the question/description... At least they don't on my phone. When using the mobile device, if you switch over to "full site mode" then the suggestions come up just fine.
In my mind, getting the suggestions to show up on the mobile version of the site should not be a hard thing to implement
While I admit it makes me lazy asking for this feature when there is a work around, the full site doesn't display nearly as well as the mobile one so it would make life a lot easier to have this simple change.
Also, what good programmer isn't lazy? :)