This may be sth. for Archaic as well

[sevu]

In debug mode it's possible to play SP campaigns in multiplayer.
In multiplayer, units are shared among add-ons, and the campaign reads them a 2nd time.
Wesnoth will tell on stderr then that it knows now multiple units with the same ID.

[sevu]

About the 2nd comment line, this is targeted to other people who might read your code.
There is no such case in the era or the campaigns. I was thinking about the lvl 4 fire elemental in War of Legends with the extra_defines, or possible new campaign writers who may want to use an advancefrom.

[doofus-01]

I recall seeing that SP campaigns could be played MP in debug, and being kind of annoyed. I still don't see the reason for that feature, but this PR looks harmless enough. Give me a few days, I'll have time to do something.
Thanks.

[sevu]

Sure, you're welcome.
I don't know what the intention was either. Maybe this is the way to play SP campaigns with enabling MP modifications.
This PR just makes the situation a little bit better, if somebody does that.
Though, I think it's not worthy to support the MP way.