diff --git a/beacon_node/beacon_chain/src/validator_monitor.rs b/beacon_node/beacon_chain/src/validator_monitor.rs
index 015efe0c64e..b0395c43bea 100644
--- a/beacon_node/beacon_chain/src/validator_monitor.rs
+++ b/beacon_node/beacon_chain/src/validator_monitor.rs
@@ -440,6 +440,16 @@ impl<T: EthSpec> ValidatorMonitor<T> {
             .and_then(|pubkey| self.validators.get(pubkey))
     }
 
+    /// Return `true` if the `validator_index` has been registered with `self`.
+    pub fn contains_validator(&self, validator_index: u64) -> bool {
+        self.indices.contains_key(&validator_index)
+    }
+
+    /// Return `true` if the automatic validator registration is enabled.
+    pub fn auto_register_enabled(&self) -> bool {
+        self.auto_register
+    }
+
     /// Returns the number of validators monitored by `self`.
     pub fn num_validators(&self) -> usize {
         self.validators.len()
diff --git a/beacon_node/http_api/src/lib.rs b/beacon_node/http_api/src/lib.rs
index d6b80510764..3847f019c03 100644
--- a/beacon_node/http_api/src/lib.rs
+++ b/beacon_node/http_api/src/lib.rs
@@ -1877,10 +1877,26 @@ pub fn serve<T: BeaconChainTypes>(
              chain: Arc<BeaconChain<T>>| {
                 blocking_json_task(move || {
                     for subscription in &subscriptions {
-                        chain
-                            .validator_monitor
-                            .write()
-                            .auto_register_local_validator(subscription.validator_index);
+                        // Assign the result from the read to a separate variable, to avoid a Rust
+                        // quirk where a read-lock obtained inside an `if` statement is for the
+                        // execution inside it.
+                        //
+                        // Checking the read-lock first helps avoid lengthy waits for the
+                        // write-lock. Although another thread may add the validator between
+                        // dropping the read-lock and obtaining the write-lock, there's no harm in
+                        // registering the same validator twice.
+                        let should_register = {
+                            let validator_monitor = chain.validator_monitor.read();
+                            validator_monitor.auto_register_enabled()
+                                && validator_monitor
+                                    .contains_validator(subscription.validator_index)
+                        };
+                        if should_register {
+                            chain
+                                .validator_monitor
+                                .write()
+                                .auto_register_local_validator(subscription.validator_index);
+                        }
 
                         let subscription = api_types::ValidatorSubscription {
                             validator_index: subscription.validator_index,