Index: export_obj.py
===================================================================
--- export_obj.py	(revision 3491)
+++ export_obj.py	(working copy)
@@ -245,7 +245,7 @@
     def veckey2d(v):
         return round(v[0], 6), round(v[1], 6)
 
-    def findVertexGroupName(face, vWeightMap):
+    def findVertexGroupsForFace(face, vWeightMap):
         """
         Searches the vertexDict to see what groups is assigned to a given face.
         We use a frequency system in order to sort out the name because a given vetex can
@@ -254,17 +254,15 @@
         frequency in descend order. The top element is the one shared by the highest number
         of vertices is the face's group
         """
-        weightDict = {}
+        vert_groups = {} # dict of vert_index: set(of group names)
         for vert_index in face.vertices:
             vWeights = vWeightMap[vert_index]
+            vert_groups[vert_index] = set()
             for vGroupName, weight in vWeights:
-                weightDict[vGroupName] = weightDict.get(vGroupName, 0.0) + weight
+                if weight > 0:
+                    vert_groups[vert_index].add(vGroupName)
+        return sorted(list(set.intersection(*[ vert_groups[v] for v in vert_groups ])))
 
-        if weightDict:
-            return max((weight, vGroupName) for vGroupName, weight in weightDict.items())[1]
-        else:
-            return '(null)'
-
     print('OBJ Export path: %r' % filepath)
 
     time1 = time.time()
@@ -454,7 +452,7 @@
                 # Retrieve the list of vertex groups
                 vertGroupNames = ob.vertex_groups.keys()
 
-                currentVGroup = ''
+                currentVGroup = []
                 # Create a dictionary keyed by face id and listing, for each vertex, the vertex groups it belongs to
                 vgroupsMap = [[] for _i in range(len(me_verts))]
                 for v_idx, v_ls in enumerate(vgroupsMap):
@@ -477,11 +475,11 @@
                 # Write the vertex group
                 if EXPORT_POLYGROUPS:
                     if ob.vertex_groups:
-                        # find what vertext group the face belongs to
-                        vgroup_of_face = findVertexGroupName(f, vgroupsMap)
+                        # find what vertext groups the face belongs to
+                        vgroup_of_face = findVertexGroupsForFace(f, vgroupsMap)
                         if vgroup_of_face != currentVGroup:
                             currentVGroup = vgroup_of_face
-                            fw('g %s\n' % vgroup_of_face)
+                            fw('g %s\n' % " ".join(vgroup_of_face))
 
                 # CHECK FOR CONTEXT SWITCH
                 if key == contextMat: