1v1

AcceptCommand

public class AcceptCommand implements CommandExecutor 
{
    private ChallengeMain plugin;
    
    public AcceptCommand(ChallengeMain plugin) {
        this.plugin = plugin;
    }
    
    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) 
    {
        if ( !(sender instanceof Player) ) return true;
        Player playerSender = (Player) sender;
        if (args.length != 1) 
        {
            sender.sendMessage( "Usage: /accept <player>" );
            return true;    
        }
        Player player = Bukkit.getPlayer( args[ 0 ] );
        if (player.isOnline()) 
        {
            
        } else 
        {
            // handle offline
        }
    }
}

ChallengeCommand

public class ChallengeCommand implements CommandExecutor 
{
    private ChallengeMain plugin;
    
    public ChallengeCommand(ChallengeMain plugin) {
        this.plugin = plugin;
    }
    
    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) 
    {
        if ( !(sender instanceof Player) ) return true;
        Player playerSender = (Player) sender;
        if (args.length != 1) 
        {
            sender.sendMessage( "Usage: /challenge <player>" );
            return true;    
        }
        Player player = Bukkit.getPlayer( args[ 0 ] );
        if ( player.isOnline() ) 
        {
            if ( !handleAdd( playerSender, player ) ) 
            {
                playerSender.sendMessage( "you have to wait to challege again!" )
            }
        } else {
            // TODO: handle offline player
        }
    }
    
    private boolean handleAdd(Player challenger, Player challenged) {
        Map<String, Long> innerMap;
        if ( plugin.getChallenges().containsKey( challenged.getName() ) )
        {
            innerMap = plugin.getChallenges().get( challenged.getName() );
            if ( innerMap.containsKey( challenger.getName() ) 
            && ( innerMap.get( challenger.getName() ) +  
                    plugin.getExpireTime() ) - ( System.currentTimeMillis() / 1000 ) > 0 ) )
            {
                    return true;
            }
        } else {
            innerMap = new HashMap<>();
            plugin.getChallenges().put( challenged.getName(), innerMap );
        }
        innerMap.put( challenger.getName(), System.currentTimeMillis() / 1000 );
        return false;
    }
}

JavaPlugin

public class ChallengeMain extends JavaPlugin 
{
    private Map<String, Map<String, Long>> challenges = new HashMap<>();
    private long expireTime = 15; // seconds
    
    @Override
    public void onEnable() 
    {
        getCommand( "challenge" ).setExecutor( new ChallengeCommand( this ) );
        getCommand( "accept" ).setExecutor( new AcceptCommand( this ) );
    }
    
    public Map<String, Map<String, Long>> getChallenges()
    {
        return challenges;
    }
    
    public long getExpireTime() 
    {
       return expireTime;
    }
}