def merge_3d_mappings(pdb_data, pdb_curated_data):
    """
    Merge PDB data from automatic Rfam mapping and a curated list.
    If a pdb_id is found in the curated list, but not found in the automatic
    mapping, it is added to a merged list.
    """
    for rfam_acc in pdb_curated_data.keys():
        curated_data = pdb_curated_data[rfam_acc]
        if rfam_acc in pdb_data:
            for pdb_id in curated_data:
                if pdb_id not in pdb_data[rfam_acc]:
                    pdb_data[rfam_acc].append(pdb_id)
        else:
            pdb_data[rfam_acc] = curated_data
    return pdb_data