Deal with multiple writing systems with same tag (#141)

Occasionally there's a project with malformed data where two different
writing systems have the same tag. We don't try to fix that data, we
just ensure that LfMerge will be able to carry on without throwing an
exception in that instance.

src/LfMerge.Core/DataConverters/ConvertLcmToMongoLexicon.cs

@@ -612,9 +612,9 @@ private Dictionary<string, LfInputSystemRecord> LcmWsToLfWs()
 				};
 
 				#if FW8_COMPAT
-				lfWsList.Add(LcmWs.Id, lfWs);
+				lfWsList[LcmWs.Id] = lfWs;
 				#else
-				lfWsList.Add(LcmWs.LanguageTag, lfWs);
+				lfWsList[LcmWs.LanguageTag] = lfWs;
 				#endif
 			}
 			return lfWsList;