Ensure gameconfig file uniqueness when reading master.games (#1859)

The extended gameconfig format reads the master gameconf file twice;
once each for the base engine and actual engine.  The file list
isn't checked for duplicates, so 'common.games.txt' is loaded in
twice, resulting in any 'common' file values potentially overriding
values listed under '#default' in other files due to
bShouldBeReadingDefault.  This happens in the case when matching
game versions by CRC (such as public game branches).

Required for #1857.

(cherry picked from commit 34c8220e5d)

core/logic/GameConfigs.cpp

@@ -810,7 +810,10 @@ public:
 				(!had_game && matched_engine) ||
 				(matched_engine && matched_game))
 			{
-				fileList->push_back(cur_file);
+				if (fileList->find(cur_file) == fileList->end())
+				{
+					fileList->push_back(cur_file);
+				}
 			}
 			state = MSTATE_MAIN;
 		}