When submitting a multipart form to a Magnolia page, all request parameters' values are inserted multiple times in the String[] of values. This is not really a problem until you have _eventName or a String[] as parameter, and in these cases you need non-duplicated values.

The problem is caused by stripes, that tries to "merge" request parameters with uri parameters: see class MergedParameterMap (at line 339 to be precise) for more info. The merge is not done by replacing parameters with the same key in the map but adding values to the values array. In the multipart request, all parameters are already set in the map by Magnolia itself, but using MergedParameterMap, stripes just re-adds them to the map each time an action is called. So if the original multipart request form has a parameter like _eventName=show, the resulting request will have _eventName=[show, show, show].

The attached patch is a modification of the original MagnoliaMultipartWrapper that does not return any parameter, because they are already in the Magnolia parsed request, avoiding parameters duplication.