diff --git a/crates/blockchain/src/lib.rs b/crates/blockchain/src/lib.rs
index 145f82d2..605327de 100644
--- a/crates/blockchain/src/lib.rs
+++ b/crates/blockchain/src/lib.rs
@@ -22,7 +22,7 @@ use spawned_concurrency::error::ActorError;
 use spawned_concurrency::protocol;
 use spawned_concurrency::tasks::{Actor, ActorRef, ActorStart, Context, Handler, send_after};
 use tokio_util::sync::CancellationToken;
-use tracing::{error, info, trace, warn};
+use tracing::{debug, error, info, trace, warn};
 
 use crate::store::StoreError;
 
@@ -173,12 +173,6 @@ pub struct BlockChainServer {
 
 impl BlockChainServer {
     async fn on_tick(&mut self, timestamp_ms: u64, ctx: &Context<Self>) {
-        // Observe tick interval duration before any processing
-        if let Some(prev_instant) = self.last_tick_instant {
-            metrics::observe_tick_interval_duration(prev_instant.elapsed());
-        }
-        self.last_tick_instant = Some(Instant::now());
-
         let genesis_time_ms = self.store.config().genesis_time * 1000;
 
         // Calculate current slot and interval from milliseconds
@@ -186,6 +180,26 @@ impl BlockChainServer {
         let slot = time_since_genesis_ms / MILLISECONDS_PER_SLOT;
         let interval = (time_since_genesis_ms % MILLISECONDS_PER_SLOT) / MILLISECONDS_PER_INTERVAL;
 
+        // Idempotency guard
+        //
+        // `slot`/`interval` come from the wall clock, but the tick cadence is driven
+        // by the monotonic clock (`tokio::sleep`). The wall clock can drift behind it
+        // inside VMs, so a tick scheduled for the next interval boundary can fire
+        // while the wall clock still reads the previous interval.
+        let tick_interval = time_since_genesis_ms / MILLISECONDS_PER_INTERVAL;
+        let store_time = self.store.time();
+
+        if store_time > 0 && tick_interval <= store_time {
+            debug!(
+                %slot,
+                %interval,
+                tick_interval,
+                store_time,
+                "Skipping already-processed tick"
+            );
+            return;
+        }
+
         // Fail fast: a state with zero validators is invalid and would cause
         // panics in proposer selection and attestation processing.
         if self.store.head_state().validators.is_empty() {
@@ -193,6 +207,13 @@ impl BlockChainServer {
             return;
         }
 
+        // Observe tick interval duration. Done after the idempotency guard so a
+        // skipped duplicate tick doesn't shorten the next real tick's sample.
+        if let Some(prev_instant) = self.last_tick_instant {
+            metrics::observe_tick_interval_duration(prev_instant.elapsed());
+        }
+        self.last_tick_instant = Some(Instant::now());
+
         // Update current slot metric
         metrics::update_current_slot(slot);