diff --git a/src/error.rs b/src/error.rs
index f1fedd7..8fd29dc 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -15,6 +15,10 @@ pub enum Error {
     #[error("Config: {0}")]
     Config(#[from] ron::error::SpannedError),
 
+    // Duplicate event
+    #[error("Duplicate")]
+    Duplicate,
+
     // End of Input
     #[error("End of input")]
     EndOfInput,
diff --git a/src/store/mod.rs b/src/store/mod.rs
index c3df13a..6fb8487 100644
--- a/src/store/mod.rs
+++ b/src/store/mod.rs
@@ -2,7 +2,8 @@ pub mod event_store;
 pub use event_store::EventStore;
 
 use crate::error::Error;
-use crate::types::Id;
+use crate::types::Event;
+use heed::types::{OwnedType, UnalignedSlice};
 use heed::{Database, Env, EnvFlags, EnvOpenOptions};
 use std::fs;
 
@@ -10,7 +11,7 @@ use std::fs;
 pub struct Store {
     events: EventStore,
     env: Env,
-    ids: Database<Id, usize>,
+    ids: Database<UnalignedSlice<u8>, OwnedType<usize>>,
 }
 
 impl Store {
@@ -37,7 +38,8 @@ impl Store {
         let mut txn = env.write_txn()?;
         let ids = env
             .database_options()
-            .types::<Id, usize>()
+            .types::<UnalignedSlice<u8>, OwnedType<usize>>()
+            .name("ids")
             .create(&mut txn)?;
         txn.commit()?;
 
@@ -51,4 +53,24 @@ impl Store {
             ids,
         })
     }
+
+    pub fn store_event(&self, event: &Event) -> Result<usize, Error> {
+        // TBD: should we validate the event?
+
+        let mut txn = self.env.write_txn()?;
+        let offset;
+
+        // Only if it doesn't already exist
+        if self.ids.get(&txn, event.id().0.as_slice())?.is_none() {
+            offset = self.events.store_event(event)?;
+
+            // Index by id
+            self.ids.put(&mut txn, event.id().0.as_slice(), &offset)?;
+            txn.commit()?;
+        } else {
+            return Err(Error::Duplicate);
+        }
+
+        Ok(offset)
+    }
 }