1. An image coding and decoding system, comprising: an image coding apparatus that codes a current block; and an image decoding apparatus that decodes the current block, wherein the image coding apparatus comprises: a first processor; and a first non-transitory memory, wherein the first processor performs, using the first non-transitory memory, first processes including: deriving a first candidate having a first motion vector that has been used to code a first block; deriving a second candidate having a second motion vector that has been used to code a second block, the second block being different from the first block; determining whether or not a total number of candidates having the first candidate and the second candidate is less than a maximum candidate number; deriving a third candidate having a third motion vector and a fourth motion vector when the total number of candidates having the first candidate and the second candidate is less than the maximum candidate number, the third motion vector being identical to the first motion vector, the fourth motion vector being identical to the second motion vector; and coding an index corresponding to a candidate having a motion vector, the candidate being one of a plurality of candidates including the first candidate, the second candidate, and the third candidate; and generating a bitstream including the coded index, wherein the maximum candidate number is used to code the index, wherein the motion vector is used to code the current block, wherein the image decoding apparatus comprises: a second processor; and a second non-transitory memory, wherein the second processor performs, using the second non-transitory memory, second processes including: deriving a fourth candidate corresponding to the first candidate; deriving a fifth candidate corresponding to the second candidate; determining whether or not a total number of candidates having the fourth candidate and the fifth candidate is less than the maximum candidate number; deriving a sixth candidate corresponding to the third candidate when the total number of candidates having the fourth candidate and the fifth candidate is less than the maximum candidate number; and decoding the coded index corresponding to the candidate having the motion vector, the candidate being one of a plurality of candidates including the fourth candidate, the fifth candidate, and the sixth candidate, wherein the maximum candidate number is used to decode the coded index, and wherein the motion vector is used to decode the current block.