diff --git a/backend/src/error.rs b/backend/src/error.rs
index 50898d9..62dfbfc 100644
--- a/backend/src/error.rs
+++ b/backend/src/error.rs
@@ -4,6 +4,7 @@ use axum::{
     Json,
 };
 use serde::Serialize;
+use serde_json::Value;
 use std::fmt::Display;
 
 pub type AppResult<T> = Result<T, AppError>;
@@ -13,6 +14,7 @@ pub struct AppError {
     status: StatusCode,
     message: String,
     code: Option<String>,
+    details: Option<Value>,
 }
 
 impl AppError {
@@ -21,6 +23,7 @@ impl AppError {
             status,
             message: message.into(),
             code: None,
+            details: None,
         }
     }
 
@@ -48,6 +51,11 @@ impl AppError {
         self.code = Some(code.into());
         self
     }
+
+    pub fn with_details(mut self, details: Value) -> Self {
+        self.details = Some(details);
+        self
+    }
 }
 
 impl IntoResponse for AppError {
@@ -56,6 +64,7 @@ impl IntoResponse for AppError {
         let body = Json(ErrorResponse {
             error: self.message,
             code: self.code,
+            details: self.details,
         });
         (status, body).into_response()
     }
@@ -66,6 +75,8 @@ struct ErrorResponse {
     error: String,
     #[serde(skip_serializing_if = "Option::is_none")]
     code: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    details: Option<Value>,
 }
 
 impl From<diesel::result::Error> for AppError {
diff --git a/backend/src/routes/documents.rs b/backend/src/routes/documents.rs
index 1525c37..d23bd3d 100644
--- a/backend/src/routes/documents.rs
+++ b/backend/src/routes/documents.rs
@@ -303,7 +303,6 @@ struct UploadRequest {
 enum UploadOutcome {
     Created(DocumentDetailResponse),
     Reused(DocumentDetailResponse),
-    Skipped { document_id: Uuid },
 }
 
 #[derive(ToSchema)]
@@ -322,7 +321,7 @@ pub struct UploadDocumentForm {
     pub correspondents: Option<Vec<CorrespondentAssignmentInput>>,
     #[schema(nullable, example = "2024-01-01T00:00:00Z")]
     pub issued_at: Option<String>,
-    #[schema(nullable)]
+    #[schema(nullable, default = true)]
     pub skip_existing: Option<bool>,
 }
 
@@ -713,7 +712,7 @@ pub async fn get_document(
     responses(
         (status = 201, description = "Document created", body = DocumentDetailResponse),
         (status = 200, description = "Existing document reused", body = DocumentDetailResponse),
-        (status = 204, description = "Upload skipped because the document already exists")
+        (status = 409, description = "Document with identical contents already exists")
     ),
     tag = "Documents"
 )]
@@ -732,7 +731,7 @@ pub async fn upload_document(
     let mut tag_ids: Vec<Uuid> = Vec::new();
     let mut correspondents: Vec<CorrespondentAssignmentInput> = Vec::new();
     let mut issued_at_override: Option<NaiveDateTime> = None;
-    let mut skip_if_existing = false;
+    let mut skip_if_existing = true;
     let mut title_override: Option<String> = None;
 
     while let Some(field) = multipart.next_field().await.map_err(|err| {
@@ -913,10 +912,6 @@ pub async fn upload_document(
             );
             (StatusCode::OK, Json(detail)).into_response()
         }
-        UploadOutcome::Skipped { document_id } => {
-            info!(document_id = %document_id, "document upload skipped by client request");
-            StatusCode::NO_CONTENT.into_response()
-        }
     };
 
     Ok(response)
@@ -2138,11 +2133,15 @@ async fn process_upload(
                 info!(
                     document_id = %document.id,
                     checksum = %checksum_hex,
-                    "upload skipped existing document due to skip flag",
+                    "upload rejected because document already exists",
+                );
+                return Err(
+                    AppError::conflict("a document with the same contents already exists")
+                        .with_code("duplicate_document")
+                        .with_details(json!({
+                            "conflict_document_id": document.id,
+                        })),
                 );
-                return Ok(UploadOutcome::Skipped {
-                    document_id: document.id,
-                });
             }
 
             if let Some(issued_at) = issued_at_override {