First, use early continue instead of nested levels.

Second, this goes to the older discussion of thew neighborhood API: it is not nice to make user code to worry about memory layout of the coordinates.

There are easy ways to make things way more explicit for the user code:

Way #1: Hide the check under BKE_subdiv_ccg_neighbour_coord_is_duplicate(). Easiest to implement, but is still not very ideal.

Way #2: Make SubdivCCGNeighbors to store SubdivCCGNeighbourCoord where the latter one is

typedef struct SubdivCCGNeighbourCoord {
  int grid_index;
  int x, y;
  bool is_duplicate;
} SubdivCCGNeighbourCoord;

The latter one makes user code explicit, and allows to simplify indexing logic in the BKE_subdiv_ccg_neighbor_coords_get().

Such refactor is something we discussed already and agreed it should happen,. yet there is new code which keeps spreading code which we want to be changed.

Move this to BKE_subdiv_ccg_neighbor_coords_free()