I noticed this issue as I tend to view Stack Overflow via the mobile site frequently:
The mobile version does not show possible duplicate questions while you're typing the question/description, while in "full site mode" 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? :)