Double signage point submitted for proof

Chia Pools
1

In the pool logs sometimes get a messages like this (dot were added for secure reason):

INFO:root:Double signage point submitted for proof: {'authentication_token': 54....21,
 'end_of_sub_slot': False,
 'harvester_id': '0x388f629408d0883a8bc2e6e24e59f556e2be0b9fd56...9b6e7d63215afee',
 'launcher_id': '0x0cf19eaa6600153db23433fc51a98060f319a9128e07f509e3c6a...80cd9',
 'proof_of_space': {'challenge': '0x1a0569a5035cab4748da2e36434f287....d9ef53131ae3af7402ee11',
                    'plot_public_key': '0x9084414609125b27555a7b54b3148235e......309a2c0d52591d395914d88c873352b9011254c701870bbadf1',
                    'pool_contract_puzzle_hash': '0xf26c5da645d8f3dad379f12de73f41......246023466f9fee31921b3cfa',
                    'pool_public_key': None,
                    'proof': '0xd8fef7fc3ccaeed03bc1a13a663a5ae795aa93ebac3b8c4d1dc10651f84d48642c9e8cbb5837e360d7d8681d2f959528bd23b987401aef1a8fca089c7cf26db4c89ebc3a1be97f4466eb0d840ef5eb2f856a13268f18d2e419b8026174e94e579e85807d1d9aa64........82201a0bea66fa67cae9d2d22956929b1f707ff9cdeb7e667e24671f460cf6a9b979e4486107bff5a9782c7db7bcd4c2aea27343e31db3f681936519753d3d39c91d68773c548cf25e4a3858a8f7ab6fb8e88c87a508e32aba5b750dc99bd26013088ac5f7c1e73178a9c5cb94b16bbd1aad5',
                    'size': 32},
 'sp_hash': '0xcdfd2925b4d3c8353bd09....2a33f8bab21e64abf8'}

What can be a reason of this and how can be solve?