i think it is necessary that. and that.

可以是：　i think it is necessary that +　主语　+　谓语 and that +　主语　+　谓语　（这里两个that 从句都是主语从句（后移的主语从句）.如 I think it is necessary that we do this quickly and that we go to sleep earlier.
也可以是：i think it is necessary that we do this quickly and go to sleep earlier.(这里只有一个that 引导的主语从句,do 与 go 为并列的谓语动词.
当作者要说的句子不长,第2个就显得垒赘.但是当作者要说的句子太长,第2个that 就显得必要,因为不然的话,读者就会比较难看清作者的句子了.
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再答： it Ϊ���дʡ�it is necessary that we do this = That we do this is necessary��������Ӿ䱻�����ˣ�