Prevent adding duplicate documents

It was possible that users would add duplicate documents via 'open with
Dangerzone'. This would lead to unexpected situations and preventing it
both in the CLI and the GUI solves those issues.

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

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."""
 

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(

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(