Maybe:

http://contextures.com/xlfaqApp.html#ReadOnly
or
http://contextures.com/xlfaqApp.html#AlreadyOpen

Both from Debra Dalgleish's site.

IanB wrote:

When I try to open a file I just had open, the following message appears -
Cannot access read-only document "name of file"
File properties do not have the read-only box ticked.

--

Dave Peterson