The macro command
/lcio/fileExists delete
will cause SLIC to delete any output file it finds with a name that matches the one it is currently using, e.g. the default name of outfile.slcio, your specified file name, or an automatically generated name.
The command line switch -x will do the same from the command line.
slic -g ./mygeom.lcdd -x -n
This setting can be changed "on the fly" during an interactive session.