What is a computer local hostname, because mine keeps on changing

A host name is the name of the computer. The local host name is the name used for Bonjour networking; what's also sometimes called mDNS or ZeroConf networking.

Common causes for duplicate-name errors can be older software (though 10.13 should be okay), actual duplicate host names on the network (booting a second system from a cloned drive can cause this), or a misconfigured network configuration.

How many network controllers are active on this Mac? Wired network and wireless network both connected and active?

Is there more than one router present, such as an ISP router and a Wi-Fi router connected to the ISP router?

It generally happens when the Mac runs a new service & thinks Macbook Pro x is busy, it can be renamed in Sys Prefs>Sharing>Computer Name.

I get the same thing when I connect and mount my EHD clone of my boot drive and it's named the same as my boot drive. I go into System/Sharing preference pane and can change it back to the original via the Edit button.

But the next time I connect the EHD the local host name gets the -2 tacked on to it.

Have you got other devices on your local network using the same name?