rc.lua

function spawn_once(command, class, tag)
   -- create move callback
   local callback
   callback = function(c)
      if c.name == class then
         awful.client.movetotag(tag, c)
         client.disconnect_signal("manage", callback)
      end
   end
   client.connect_signal("manage", callback)
   -- now check if not already running!
   local findme = command
   local firstspace = findme:find(" ")
   if firstspace then
      findme = findme:sub(0, firstspace-1)
   end
   -- finally run it
   awful.util.spawn_with_shell("pgrep -u $USER -x " .. findme .. " > /dev/null || (" .. command .. ")")
end