Duplication when pasting text

I have a table with 50 row, 4 columns. Another program creates a text file which I copy. I then highlight all of the first column, and paste. The problem is this: suppose there were 20 items in the text file. Instead of pasting 20, it pastes the 20, then the 20 again, then the first 10 again! So even though I only copied ans pasted 20, all 50 cells in the first row are filled.

Of course if I only highlighted 15 rows, then only the first 15 get pasted.

What gives here??? I don't want to create table from text, as I would lose my saved headers (right??)

Using 2007

Thanks