redefinition of 'capi_checksum256' error

[learnforpractice]

The following code:

#include <eosio/system.hpp>
#include <eosio/crypto_ext.hpp>

raise the following error:

include/eosiolib/core/eosio/crypto_ext.hpp:18:47: error: redefinition of 'capi_checksum256'
         struct __attribute__((aligned (16))) capi_checksum256 { uint8_t hash[32]; };
                                              
include/eosiolib/contracts/eosio/system.hpp:17:45: note: previous definition is here
      struct  __attribute__((aligned (16))) capi_checksum256 {