diff --git a/crates/store_sqlite/src/addressbook_store.rs b/crates/store_sqlite/src/addressbook_store.rs
index 0ca74df..9233ef7 100644
--- a/crates/store_sqlite/src/addressbook_store.rs
+++ b/crates/store_sqlite/src/addressbook_store.rs
@@ -275,20 +275,30 @@ impl AddressbookStore for SqliteStore {
         principal: String,
         addressbook_id: String,
         object: AddressObject,
-        // TODO: implement
         overwrite: bool,
     ) -> Result<(), rustical_store::Error> {
         let mut tx = self.db.begin().await.map_err(crate::Error::from)?;
 
         let (object_id, vcf) = (object.get_id(), object.get_vcf());
 
-        sqlx::query!(
+        (if overwrite {
+            sqlx::query!(
             "REPLACE INTO addressobjects (principal, addressbook_id, id, vcf) VALUES (?, ?, ?, ?)",
             principal,
             addressbook_id,
             object_id,
             vcf
         )
+        } else {
+            // If the object already exists a database error is thrown and handled in error.rs
+            sqlx::query!(
+            "INSERT INTO addressobjects (principal, addressbook_id, id, vcf) VALUES (?, ?, ?, ?)",
+            principal,
+            addressbook_id,
+            object_id,
+            vcf
+        )
+        })
         .execute(&mut *tx)
         .await
         .map_err(crate::Error::from)?;
diff --git a/crates/store_sqlite/src/calendar_store.rs b/crates/store_sqlite/src/calendar_store.rs
index a2305cf..aed9899 100644
--- a/crates/store_sqlite/src/calendar_store.rs
+++ b/crates/store_sqlite/src/calendar_store.rs
@@ -223,20 +223,29 @@ impl CalendarStore for SqliteStore {
         principal: String,
         cal_id: String,
         object: CalendarObject,
-        // TODO: implement
         overwrite: bool,
     ) -> Result<(), Error> {
         let mut tx = self.db.begin().await.map_err(crate::Error::from)?;
 
         let (object_id, ics) = (object.get_id(), object.get_ics());
 
-        sqlx::query!(
-            "REPLACE INTO calendarobjects (principal, cal_id, id, ics) VALUES (?, ?, ?, ?)",
-            principal,
-            cal_id,
-            object_id,
-            ics
-        )
+        (if overwrite {
+            sqlx::query!(
+                "REPLACE INTO calendarobjects (principal, cal_id, id, ics) VALUES (?, ?, ?, ?)",
+                principal,
+                cal_id,
+                object_id,
+                ics
+            )
+        } else {
+            sqlx::query!(
+                "INSERT INTO calendarobjects (principal, cal_id, id, ics) VALUES (?, ?, ?, ?)",
+                principal,
+                cal_id,
+                object_id,
+                ics
+            )
+        })
         .execute(&mut *tx)
         .await
         .map_err(crate::Error::from)?;
diff --git a/crates/store_sqlite/src/error.rs b/crates/store_sqlite/src/error.rs
index b93ad4f..b372696 100644
--- a/crates/store_sqlite/src/error.rs
+++ b/crates/store_sqlite/src/error.rs
@@ -11,6 +11,13 @@ impl From<sqlx::Error> for Error {
     fn from(value: sqlx::Error) -> Self {
         match value {
             sqlx::Error::RowNotFound => Error::StoreError(rustical_store::Error::NotFound),
+            sqlx::Error::Database(err) => {
+                if err.is_unique_violation() {
+                    Error::StoreError(rustical_store::Error::AlreadyExists)
+                } else {
+                    Error::SqlxError(sqlx::Error::Database(err))
+                }
+            }
             err => Error::SqlxError(err),
         }
     }