Fix localization

Author: koppor

src/main/java/org/jabref/gui/preferences/customentrytypes/CustomEntryTypesTab.java

@@ -161,7 +161,7 @@ private void setupFieldsTable() {
                             fieldViewModel.nameProperty().getValue().equalsIgnoreCase(newFieldValue));
 
                     if (fieldExists) {
-                        dialogService.notify(Localization.lang("Duplicate fields: You added field \"%0\" twice. Only one will be kept.", newFieldValue));
+                        dialogService.notify(Localization.lang("Unable to change field name. \"%0\" already in use.", newFieldValue));
                         event.getTableView().edit(-1, null);
                         event.getTableView().refresh();
                     } else {

src/main/resources/l10n/JabRef_en.properties

@@ -275,7 +275,7 @@ duplicate\ removal=duplicate removal
 
 Duplicate\ fields=Duplicate fields
 
-Duplicate\ fields\:\ You\ added\ field\ "%0"\ twice.\ Only\ one\ will\ be\ kept.=Duplicate fields: You added field "%0" twice. Only one will be kept.
+Unable\ to\ change\ field\ name\.\ "%0"\ already\ in\ use.=Unable to change field name. "%0" already in use.
 
 Duplicate\ string\ name=Duplicate string name