I've recently bought an Arabic Oud and started studying how to play it (it's a very fun instrument to play). One thing I don't understand, though, is why I have not seen anyone using the lowest pitch string as a double course, when there usually is a peg avaiable for that.
It would appear from several sources that the stringing of the oud depends heavily on the make of your oud be it Iraqi, Syrian, Egyptian, etc. It would seem that the stringing setup is a matter of local preference. A good source to refer to would be the stringing and tuning page at Oudcafe.com :
The Syrian oud often has eleven strings and some common tunings are:
C F A d g c
D G A d g c
C E A d g c
F A d g c f (usually all double courses with this tuning - 12 strings)
Egyptian ouds usually differ most from Syrian ouds not in tuning, but in their general tone. Often though you will find Egyptian players using only five pairs of strings, in effect removing the lowest 'drone string' from the first three tunings above:
F A d g c
G A d g c
E A d g c
When the Egyptian oud is used with six courses, it can bes as seen above in the Syrian oud tunings.
Iraqi style ouds are often called 'Bachir' ouds because they are rightly attributed to the famous brothers Jameel and Munir Bachir, who were huge forces in creating a new school of oud playing and oud making. These are completey different in that they have floating bridges and the strings attach at the base of the oud, not on the bridge. They often have seven or eight courses (13-15 individual strings). Also, the length of these ouds is usually closer to the Turkish design, though in other respects they do not really resemble Turkish ouds.
C D g c f F (note the bass F 'drone string' under the most treble string pair)
F C D g c f
According to this random person on the Internet, Arabic Ouds are sometimes available in 10 or 12 string varieties.
According to this string reseller, ouds can have "11 or 12 strings in 5 or 6 courses".
And finally this site shows a diagram of an 11-string oud but also says "in some ouds you can find 12 strings in the same corresponding order"
So you probably have a 12-string oud. That said, leaving off a string is probably not a problem if you want to play it like an 11-string and have it sound like an 11-string.