Ah OK.
I guess you want to use the same data in your Dataset?
If so, you could just fake a new length and use a modulo operation in index to avoid an out of range error:
class BarDataset(Dataset):
    def __init__(self):
        self.data = list(np.random.rand(9))
        pass

    def __getitem__(self, idx):
        # some ops
        idx = idx % len(self.data)
        out = np.array([self.data[idx] * 2, self.data[idx] * 3])
        return out

    def __len__(self):
        return 2 * len(self.data)