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int month, print_client, records_printed = 0;
int distinc[LENGTH];
// получение month, создание файла и что-то ещё...
for (int ci = 0; ci < clen; ci++)
{
    print_client = 0;
    for (int i = 0; i < LENGTH; i++)
        if (distinc[i])
            distinc[i] = 0;
        else break;
    for (int ri = 0; ri < rlen; ri++) {
        if (clients[ci].number == records[ri].number && records[ri].cdate.month == month)
        {
            if (!print_client)
                fprintf(file, "%s, %lli:\n", clients[ci].fullname, clients[ci].number);
            ++print_client;
            int service = records[ri].service;
            for (int i = 0; i < LENGTH; i++)
            {
                if (distinc[i])
                {
                    if (distinc[i] == service)
                        service = 0;
                }
                else
                {
                    distinc[i] = service;
                    break;
                }
            }
            if (service)
            {
                for (int si = 0; si < slen; si++)
                {
                    if (service == services[si].code)
                    {
                        fprintf(file, "\t%s\n", services[si].name);
                        ++records_printed;
                        break;
                    }
                }
            }
        }
    }
}