is a variable defined in `
Its value is
If non-nil, Dired always finds a directory in a buffer of its own.
If nil, Dired finds the directory as a subdirectory in some other buffer
if it is present as one.
If there are several Dired buffers for a directory, the most recently
used is chosen.
Dired avoids switching to the current buffer, so that if you have
a normal and a wildcard buffer for the same directory, C-x d will
toggle between those two.
You can customize this variable.