Please tell me how to write the auto sequential number of index 0th id to Excel after Marge 2 DataTables

As for the topic,

Once in the Build DataTable, “id”, “Title”, “URL”, “Description”
We created DataTable 1 and DataTable 2 with the same column name.
Since Merge DataTable is added as it is to the number of Autoincrement of id,
For now, id is not automatically sequential number, only column name is done.

What I want to do is, ultimately
Comparison of rows of column names in DataTable 1 and 2.

Considering the speed of processing,
For each loop, not
I think whether deletion of duplicate rows is good after Merge,
We have done so far.
But I do not know if this is the best way to handle.
Even if it becomes a DataTable to 10000 lines in the future,
I want to make processing finish quickly.

Because it is a beginner,
I would be pleased if you could tell me carefully if possible.
Thank you very much.