Good luck for correcting them, there are tons of those faulty mp's all over the place in NL's ;-) I wouldnt spend too much time for this, better ignore it. The guy who imported those 3D-landuse shapes almost risked his health :-/
I cannot think of any. It should also not make any sense for the same way to be in different roles in a multipolygon relation.
(In a route relation, you could have the same way twice, in role=forward and role=backward. You might even have a way multiple times, if the route is visiting points A, B, A, C, A.)
The JOSM relation editor is highlighting duplicate relation members. I think that it would be good to issue a log message for the duplicate multipolygon members, so that the data can be corrected (simplified).
Marko