I am unable to download multiple books by the same author

[footloose]

In my pc, I have two files containing many thousands of books in each. Every time I try to download the multiple books from the file of an individual author, it will only allow me to select one book. Further attempts to download more are greeted with the message that (name of first book chosen) already exists in Calibre, do I wish to replace it. No amount of fiddling about (I am not a tech wizard) allows me to circumvent this problem. Can anybody please help?