diff --git a/CHANGELOG.md b/CHANGELOG.md
index fe7c548e02f..3755fa4c39c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,7 @@ Note that this project **does not** adhere to [Semantic Versioning](http://semve
 - We fixed an issue where clicking on headings in the entry preview could lead to an exception. [#8292](https://github.com/JabRef/jabref/issues/8292)
 - We fixed an issue where IntegrityCheck used the system's character encoding instead of the one set by the library or in preferences [#8022](https://github.com/JabRef/jabref/issues/8022)
 - We fixed an issue about empty metadata in library properties when called from the right click menu. [#8358](https://github.com/JabRef/jabref/issues/8358)
+- We fixed an issue where someone could add a duplicate field in the customize entry type dialog. [#8194](https://github.com/JabRef/jabref/issues/8194)
 - We fixed a typo in the library properties tab: "String constants". There, one can configure [BibTeX string constants](https://docs.jabref.org/advanced/strings).
 
 ### Removed
diff --git a/src/main/java/org/jabref/gui/customentrytypes/CustomEntryTypeDialogViewModel.java b/src/main/java/org/jabref/gui/customentrytypes/CustomEntryTypeDialogViewModel.java
index d063c3c0994..ab51c2f7390 100644
--- a/src/main/java/org/jabref/gui/customentrytypes/CustomEntryTypeDialogViewModel.java
+++ b/src/main/java/org/jabref/gui/customentrytypes/CustomEntryTypeDialogViewModel.java
@@ -16,6 +16,7 @@
 import javafx.collections.ObservableList;
 import javafx.util.StringConverter;
 
+import org.jabref.gui.DialogService;
 import org.jabref.logic.l10n.Localization;
 import org.jabref.model.database.BibDatabaseMode;
 import org.jabref.model.entry.BibEntryType;
@@ -59,14 +60,16 @@ public Field fromString(String string) {
 
     private final PreferencesService preferencesService;
     private final BibEntryTypesManager entryTypesManager;
+    private final DialogService dialogService;
 
     private final Validator entryTypeValidator;
     private final Validator fieldValidator;
 
-    public CustomEntryTypeDialogViewModel(BibDatabaseMode mode, PreferencesService preferencesService, BibEntryTypesManager entryTypesManager) {
+    public CustomEntryTypeDialogViewModel(BibDatabaseMode mode, PreferencesService preferencesService, BibEntryTypesManager entryTypesManager, DialogService dialogService) {
         this.mode = mode;
         this.preferencesService = preferencesService;
         this.entryTypesManager = entryTypesManager;
+        this.dialogService = dialogService;
 
         addAllTypes();
 
@@ -126,7 +129,14 @@ public String toString() {
     public void addNewField() {
         Field field = newFieldToAdd.getValue();
         FieldViewModel model = new FieldViewModel(field, true, FieldPriority.IMPORTANT);
-        this.selectedEntryType.getValue().addField(model);
+        ObservableList<FieldViewModel> entryFields = this.selectedEntryType.getValue().fields();
+        boolean fieldExists = entryFields.stream().anyMatch(fieldViewModel -> fieldViewModel.fieldName().getValue().equals(field.getDisplayName()));
+
+        if (!fieldExists) {
+            this.selectedEntryType.getValue().addField(model);
+        } else {
+            dialogService.showWarningDialogAndWait(Localization.lang("Duplicate fields"), Localization.lang("Warning: You added field \"%0\" twice. Only one will be kept.", field.getDisplayName()));
+        }
         newFieldToAddProperty().setValue(null);
     }
 
diff --git a/src/main/java/org/jabref/gui/customentrytypes/CustomizeEntryTypeDialogView.java b/src/main/java/org/jabref/gui/customentrytypes/CustomizeEntryTypeDialogView.java
index cc85bf98c87..7a4973cf8e2 100644
--- a/src/main/java/org/jabref/gui/customentrytypes/CustomizeEntryTypeDialogView.java
+++ b/src/main/java/org/jabref/gui/customentrytypes/CustomizeEntryTypeDialogView.java
@@ -96,7 +96,7 @@ private void initialize() {
         // As the state manager gets injected it's not available in the constructor
         this.localDragboard = stateManager.getLocalDragboard();
 
-        viewModel = new CustomEntryTypeDialogViewModel(mode, preferencesService, entryTypesManager);
+        viewModel = new CustomEntryTypeDialogViewModel(mode, preferencesService, entryTypesManager, dialogService);
         setupTable();
 
         addNewEntryTypeButton.disableProperty().bind(viewModel.entryTypeValidationStatus().validProperty().not());
diff --git a/src/main/resources/l10n/JabRef_en.properties b/src/main/resources/l10n/JabRef_en.properties
index 51561772656..2b7f8ecc5bd 100644
--- a/src/main/resources/l10n/JabRef_en.properties
+++ b/src/main/resources/l10n/JabRef_en.properties
@@ -275,6 +275,8 @@ Downloaded\ website\ as\ an\ HTML\ file.=Downloaded website as an HTML file.
 
 duplicate\ removal=duplicate removal
 
+Duplicate\ fields=Duplicate fields
+
 Duplicate\ string\ name=Duplicate string name
 
 Duplicates\ found=Duplicates found
@@ -902,6 +904,8 @@ Warning=Warning
 
 Warnings=Warnings
 
+Warning\:\ You\ added\ field\ "%0"\ twice.\ Only\ one\ will\ be\ kept.=Warning: You added field "%0" twice. Only one will be kept.
+
 web\ link=web link
 
 What\ do\ you\ want\ to\ do?=What do you want to do?