Local hostname is already in use on this network?
To analyze what might be this ghost network host, the command to use is a command line (which you have to run in Terminal or xterm):

dns-sd -G v4 ----Macbook-pro.local


If this name really exists within your accessible Wi-Fi network, then
you will get its IP address within the last column of output of this command as in this example:

DATE: ---Sat 09 Dec 2017---
 0:42:42.548  ...STARTING...
Timestamp     A/R Flags if Hostname                               Address                                      TTL
 0:42:42.856  Add     2  6 ----Macbook-pro.local                  10.0.2.23                                    120


If you just get one IP address, then this error message was probably an internal bug within the MacOS way to manage its DNS cache.

If you get two different IP addresses, then you have an uninvited host within your network and behind your firewall (i.e. directly on the Wi-Fi
part). This might be one of your neighbor. To continue on its correct
identification, you will have to use methods which are considered as direct attacks on someone's computer.

For example, you might use iStumbler to physically locate it and then have a chance to interview the owner of this new visitor of your network.