desktop-restore-reuses-frames

desktop-restore-reuses-frames is a variable defined in `desktop.el'.
Its value is

t

Documentation:
If t, restoring frames reuses existing frames.
If nil, deletes existing frames.
If `keep', keeps existing frames and does not reuse them.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.4 of Emacs.