Excel prompted duplicate files loaded !!!

[JJ]

Dear Expert,

when I opened a xls file by double clicking the file
directly, the excel
prompted "A document with the name "abc.xls" is already
open. You cannot open
two documents with the same name, even if the documents
are in different
folders."
Why ?

If I opened Excel first, then open file by using File ->
Open, this message
would not prompt.
Why ?

Then I searched the file "abc.xls", the search engine
shown there had two
entries displayed having identical name and path, why ?

So strange.

Thanks for help !