Mess of data when saving custom checkbox field

Hi,

when saving a listing a custom checkbox field with 3 checked options saves the following into the DB (unserialized view):

array (
  0 => 'Kleintiere',
  1 => 'Reptilien',
  2 => 'Gefluegel',
  3 => 'Kleintiere',
  4 => 'Reptilien',
  5 => 'Gefluegel',
  6 => 'Kleintiere',
  7 => 'Reptilien',
  8 => 'Gefluegel',
  9 => 'Kleintiere',
  10 => 'Reptilien',
  11 => 'Gefluegel',
  12 => 'Kleintiere',
  13 => 'Reptilien',
  14 => 'Gefluegel',
  15 => 'Kleintiere',
  16 => 'Reptilien',
  17 => 'Gefluegel',
  18 => 'Kleintiere',
  19 => 'Reptilien',
  20 => 'Gefluegel',
  21 => 'Kleintiere',
  22 => 'Reptilien',
  23 => 'Gefluegel',
  24 => 'Kleintiere',
  25 => 'Reptilien',
  26 => 'Gefluegel',
  27 => 'Kleintiere',
  28 => 'Reptilien',
  29 => 'Gefluegel',
  30 => 'Kleintiere',
  31 => 'Reptilien',
  32 => 'Gefluegel',
  33 => 'Kleintiere',
  34 => 'Reptilien',
  35 => 'Gefluegel',
  36 => 'Kleintiere',
  37 => 'Reptilien',
  38 => 'Gefluegel',
  39 => 'Kleintiere',
  40 => 'Reptilien',
  41 => 'Gefluegel',
  42 => 'Kleintiere',
  43 => 'Reptilien',
  44 => 'Gefluegel',
)

Why does it save the checked values multiple times? On bigger forms there is a lot of waste of storage.

Thanks in advance