Avoid triggering an assertion when a duplicated side definition exists

(bug #7252).

changelog

@@ -24,6 +24,8 @@ Version 1.3.13+svn:
  * WML engine:
    * Fixed a problem where two [recall]s in a SP game could get an out of sync
      recruitment
+   * avoid triggering an assertion when a duplicated side definition exists
+     (bug #7252)
  * miscellaneous and bug fixes:
    * when loading a unit some traits didn't get applied correctly but got
      fixed in a later state. This could lead to some units not leveling 

src/config_adapter.cpp

@@ -23,8 +23,10 @@
 #include "game_errors.hpp"
 #include "game_preferences.hpp"
 #include "gamestatus.hpp"
+#include "gettext.hpp"
 #include "log.hpp"
 #include "team.hpp"
+#include "wml_exception.hpp"
 
 #define LOG_NG LOG_STREAM(info, engine)
 #define ERR_NG LOG_STREAM(err, engine)
@@ -148,6 +150,11 @@ void get_player_info(const config& cfg, game_state& gamestate,
 				lexical_cast<std::string>(side) + ".");
 		}
 
+		utils::string_map symbols;
+		symbols["side"] = lexical_cast<std::string>(side);
+		VALIDATE(units.count(start_pos) == 0, 
+			 t_string(vgettext("Duplicate side definition for side '$side|' found.", symbols)));
+
 		new_unit.new_turn();
 		units.add(new std::pair<gamemap::location,unit>(map.starting_position(new_unit.side()), new_unit));
 		LOG_NG << "initializing side '" << cfg["side"] << "' at "