[GRASS-user] v.patch -a: how to interpret message "Line -58728 already has area/isle -1611 to left"

[Moritz Lennert]

I am trying to patch together a series of vector maps, using v.patch -a 
(apparently without -a, category values are not retained).  These maps 
contain polygons that were extracted from the same base map (but in each 
map there are different polygons than in the others), treated with 
different intermediary steps, to then be all put back together in a new 
vector map.

However, for one of these maps, when I try to add it, I get:

WARNING: Line -58728 already has area/isle -1611 to left
ERROR: Unable to add isle (map closed, topo saved)

How should I interpret this message ?

Moritz