diff --git a/dangerzone/document.py b/dangerzone/document.py
index aa78de3..36145a9 100644
--- a/dangerzone/document.py
+++ b/dangerzone/document.py
@@ -195,3 +195,8 @@ class Document:
     def mark_as_safe(self) -> None:
         log.debug(f"Marking doc {self.id} as 'safe'")
         self.state = Document.STATE_SAFE
+
+    def __eq__(self, other: object) -> bool:
+        if not isinstance(other, Document):
+            return False
+        return self.input_filename == other.input_filename
diff --git a/dangerzone/errors.py b/dangerzone/errors.py
index 5e38015..0f8a557 100644
--- a/dangerzone/errors.py
+++ b/dangerzone/errors.py
@@ -14,6 +14,13 @@ class DocumentFilenameException(Exception):
     """Exception for document-related filename errors."""
 
 
+class AddedDuplicateDocumentException(DocumentFilenameException):
+    """Exception for a document is added twice."""
+
+    def __init__(self) -> None:
+        super().__init__("A document was added twice")
+
+
 class InputFileNotFoundException(DocumentFilenameException):
     """Exception for when an input file does not exist."""
 
diff --git a/dangerzone/gui/main_window.py b/dangerzone/gui/main_window.py
index a7e29aa..11d36ea 100644
--- a/dangerzone/gui/main_window.py
+++ b/dangerzone/gui/main_window.py
@@ -240,12 +240,22 @@ class ContentWidget(QtWidgets.QWidget):
 
     def documents_selected(self, new_docs: List[Document]) -> None:
         if not self.conversion_started:
-            for doc in new_docs:
-                self.dangerzone.add_document(doc)
+            for doc in new_docs.copy():
+                try:
+                    self.dangerzone.add_document(doc)
+                except errors.AddedDuplicateDocumentException:
+                    new_docs.remove(doc)
+                    Alert(
+                        self.dangerzone,
+                        message=f"Document '{doc.input_filename}' has already been added for conversion.",
+                        has_cancel=False,
+                    ).exec_()
 
             self.doc_selection_widget.hide()
             self.settings_widget.show()
-            self.documents_added.emit(new_docs)
+
+            if len(new_docs) > 0:
+                self.documents_added.emit(new_docs)
 
         else:
             Alert(
diff --git a/dangerzone/logic.py b/dangerzone/logic.py
index 1fbe2cf..97cda84 100644
--- a/dangerzone/logic.py
+++ b/dangerzone/logic.py
@@ -12,7 +12,7 @@ from typing import Callable, List, Optional
 import appdirs
 import colorama
 
-from . import container
+from . import container, errors
 from .document import Document
 from .settings import Settings
 from .util import get_resource_path
@@ -51,6 +51,8 @@ class DangerzoneCore(object):
         self.add_document(doc)
 
     def add_document(self, doc: Document) -> None:
+        if doc in self.documents:
+            raise errors.AddedDuplicateDocumentException()
         self.documents.append(doc)
 
     def convert_documents(