diff --git a/services/identity/src/client_service.rs b/services/identity/src/client_service.rs
--- a/services/identity/src/client_service.rs
+++ b/services/identity/src/client_service.rs
@@ -830,18 +830,11 @@
       &message.signature,
     )?;
 
-    let mut filtered_usernames = Vec::new();
-
-    for username in usernames {
-      if !self
-        .client
-        .username_taken(username.clone())
-        .await
-        .map_err(handle_db_error)?
-      {
-        filtered_usernames.push(username);
-      }
-    }
+    let filtered_usernames = self
+      .client
+      .filter_out_taken_usernames(usernames)
+      .await
+      .map_err(handle_db_error)?;
 
     self
       .client
diff --git a/services/identity/src/database.rs b/services/identity/src/database.rs
--- a/services/identity/src/database.rs
+++ b/services/identity/src/database.rs
@@ -1,5 +1,5 @@
 use constant_time_eq::constant_time_eq;
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet};
 use std::fmt::{Display, Formatter, Result as FmtResult};
 use std::str::FromStr;
 use std::sync::Arc;
@@ -485,6 +485,23 @@
     Ok(result.is_some())
   }
 
+  pub async fn filter_out_taken_usernames(
+    &self,
+    usernames: Vec<String>,
+  ) -> Result<Vec<String>, Error> {
+    let db_usernames = self.get_all_usernames().await?;
+
+    let db_usernames_set: HashSet<String> = db_usernames.into_iter().collect();
+    let usernames_set: HashSet<String> = usernames.into_iter().collect();
+
+    let available_usernames: Vec<String> = usernames_set
+      .difference(&db_usernames_set)
+      .cloned()
+      .collect();
+
+    Ok(available_usernames)
+  }
+
   async fn get_user_from_user_info(
     &self,
     user_info: String,
@@ -631,12 +648,12 @@
       .map_err(|e| Error::AwsSdk(e.into()))
   }
 
-  pub async fn get_users(&self) -> Result<Vec<String>, Error> {
+  async fn get_all_usernames(&self) -> Result<Vec<String>, Error> {
     let scan_output = self
       .client
       .scan()
       .table_name(USERS_TABLE)
-      .projection_expression(USERS_TABLE_PARTITION_KEY)
+      .projection_expression(USERS_TABLE_USERNAME_ATTRIBUTE)
       .send()
       .await
       .map_err(|e| Error::AwsSdk(e.into()))?;
@@ -644,12 +661,12 @@
     let mut result = Vec::new();
     if let Some(attributes) = scan_output.items {
       for mut attribute in attributes {
-        let id = parse_string_attribute(
-          USERS_TABLE_PARTITION_KEY,
-          attribute.remove(USERS_TABLE_PARTITION_KEY),
-        )
-        .map_err(Error::Attribute)?;
-        result.push(id);
+        if let Ok(username) = parse_string_attribute(
+          USERS_TABLE_USERNAME_ATTRIBUTE,
+          attribute.remove(USERS_TABLE_USERNAME_ATTRIBUTE),
+        ) {
+          result.push(username);
+        }
       }
     }
     Ok(result)
@@ -683,49 +700,31 @@
     &self,
     usernames: Vec<String>,
   ) -> Result<(), Error> {
-    let mut write_requests = vec![];
-
-    for username in usernames {
-      let item: HashMap<String, AttributeValue> = vec![(
-        RESERVED_USERNAMES_TABLE_PARTITION_KEY.to_string(),
-        AttributeValue::S(username),
-      )]
-      .into_iter()
-      .collect();
-
-      let write_request = WriteRequest::builder()
-        .put_request(PutRequest::builder().set_item(Some(item)).build())
-        .build();
+    // A single call to BatchWriteItem can consist of up to 25 operations
+    for usernames_chunk in usernames.chunks(25) {
+      let write_requests = usernames_chunk
+        .iter()
+        .map(|username| {
+          let item: HashMap<String, AttributeValue> = vec![(
+            RESERVED_USERNAMES_TABLE_PARTITION_KEY.to_string(),
+            AttributeValue::S(username.to_string()),
+          )]
+          .into_iter()
+          .collect();
 
-      write_requests.push(write_request);
-    }
+          WriteRequest::builder()
+            .put_request(PutRequest::builder().set_item(Some(item)).build())
+            .build()
+        })
+        .collect();
 
-    loop {
-      let output = self
+      self
         .client
         .batch_write_item()
         .request_items(RESERVED_USERNAMES_TABLE, write_requests)
         .send()
         .await
         .map_err(|e| Error::AwsSdk(e.into()))?;
-
-      let unprocessed_items_map = match output.unprocessed_items() {
-        Some(map) => map,
-        None => break,
-      };
-
-      let unprocessed_requests =
-        match unprocessed_items_map.get(RESERVED_USERNAMES_TABLE) {
-          Some(requests) => requests,
-          None => break,
-        };
-
-      info!(
-        "{} unprocessed items, retrying...",
-        unprocessed_requests.len()
-      );
-
-      write_requests = unprocessed_requests.to_vec();
     }
 
     info!("Batch write item to reserved usernames table succeeded");