Handle units with the same id at different points in a scenario

[stevecotton]

Found by @Toranks,

That is the case of UTBS S08, where the messenger knight id=messenger is always generated with the same id, although there are never two alive at the same time. It's a conflict with a mod that's the real problem, not the scenario.

Context is last night's discussion in the Discord #development / IRC #wesnoth-dev channel.

[Toranks]

If the idea is to prevent two units with the same id from being added to the array of wounded units, the solution is as simple as three extra lines here:

#define NFW_CATCH_LAST_BREATH
     [filter]
         [not]
             id=$nfw_exclude_unit_id
             [or]
                 find_in=nfw_wounded
             [/or]
         [/not]
     [/filter]