Just use the same file number For example, in chapter 1, you mentioned the reference of 16, which is generally marked as follows:

1.3.3 xxxxxx

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx[ 16].

In Chapter 3, when you mention the 16 document again, it is the same annotation method:

3.2. 1 xxxxxxxxxxxx

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx[42].xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx[ 16].