diff --git a/src/services/idempotency.service.ts b/src/services/idempotency.service.ts
new file mode 100644
index 0000000..ceb180b
--- /dev/null
+++ b/src/services/idempotency.service.ts
@@ -0,0 +1,114 @@
+import type { TransactionType } from '../types'
+
+interface IdempotencyRecord {
+  idempotencyKey: string
+  xdrHash: string
+  transactionHash: string
+  type: TransactionType
+  createdAt: string // ISO 8601
+}
+
+const STORAGE_KEY = 'stepfi-idempotency'
+const TTL_MS = 24 * 60 * 60 * 1000 // 24 hours
+
+function getRecords(): IdempotencyRecord[] {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY)
+    if (!raw) return []
+    const parsed: unknown = JSON.parse(raw)
+    if (!Array.isArray(parsed)) return []
+    return parsed.filter(
+      (r): r is IdempotencyRecord =>
+        typeof r === 'object' &&
+        r !== null &&
+        typeof (r as IdempotencyRecord).idempotencyKey === 'string' &&
+        typeof (r as IdempotencyRecord).xdrHash === 'string' &&
+        typeof (r as IdempotencyRecord).transactionHash === 'string' &&
+        typeof (r as IdempotencyRecord).type === 'string' &&
+        typeof (r as IdempotencyRecord).createdAt === 'string'
+    )
+  } catch {
+    return []
+  }
+}
+
+function saveRecords(records: IdempotencyRecord[]): void {
+  try {
+    localStorage.setItem(STORAGE_KEY, JSON.stringify(records))
+  } catch {
+    // localStorage full or unavailable — silently drop
+  }
+}
+
+function pruneExpired(records: IdempotencyRecord[]): IdempotencyRecord[] {
+  const now = Date.now()
+  return records.filter((r) => {
+    const createdAt = new Date(r.createdAt).getTime()
+    return now - createdAt < TTL_MS
+  })
+}
+
+async function hashXdr(xdr: string): Promise<string> {
+  const encoder = new TextEncoder()
+  const data = encoder.encode(xdr)
+
+  if (typeof crypto !== 'undefined' && crypto.subtle) {
+    const hashBuffer = await crypto.subtle.digest('SHA-256', data)
+    const hashArray = Array.from(new Uint8Array(hashBuffer))
+    return hashArray.map((b) => b.toString(16).padStart(2, '0')).join('')
+  }
+
+  throw new Error('Web Crypto API not available for XDR hashing.')
+}
+
+export const idempotencyService = {
+  /** Generate a UUID v4 idempotency key. */
+  generateKey(): string {
+    return crypto.randomUUID()
+  },
+
+  /**
+   * Hash an XDR string for deduplication.
+   * Returns a hex-encoded SHA-256 hash.
+   */
+  hashXdr,
+
+  /**
+   * Check if an XDR hash has already been submitted.
+   * Returns the existing record or null.
+   * Expired records (> 24 hours) are pruned before lookup.
+   */
+  findExisting(xdrHash: string): IdempotencyRecord | null {
+    const records = pruneExpired(getRecords())
+    // Persist the pruned list so expired entries don't accumulate
+    saveRecords(records)
+    return records.find((r) => r.xdrHash === xdrHash) ?? null
+  },
+
+  /**
+   * Check if an idempotency key has already been submitted.
+   * Returns the existing record or null.
+   * Expired records are pruned before lookup.
+   */
+  findByKey(idempotencyKey: string): IdempotencyRecord | null {
+    const records = pruneExpired(getRecords())
+    saveRecords(records)
+    return records.find((r) => r.idempotencyKey === idempotencyKey) ?? null
+  },
+
+  /**
+   * Store a completed transaction record for future deduplication.
+   */
+  store(record: IdempotencyRecord): void {
+    const records = pruneExpired(getRecords())
+    records.push(record)
+    saveRecords(records)
+  },
+
+  /**
+   * Clear all idempotency records (for testing / manual reset).
+   */
+  clear(): void {
+    localStorage.removeItem(STORAGE_KEY)
+  },
+}
diff --git a/src/services/transactions.service.ts b/src/services/transactions.service.ts
index e05e537..58a3545 100644
--- a/src/services/transactions.service.ts
+++ b/src/services/transactions.service.ts
@@ -1,4 +1,5 @@
 import { api } from './api'
+import { idempotencyService } from './idempotency.service'
 import type { SubmittedTransaction, TransactionType } from '../types'
 
 export const transactionsService = {
@@ -6,17 +7,57 @@ export const transactionsService = {
    * Submits a signed XDR transaction to the Stellar network via the backend.
    *
    * Backend contract (POST /transactions/submit, SubmitTransactionRequestDto):
-   * body is `{ xdr, type }` — the field is named `xdr`, not `signedXdr`.
+   * body is `{ xdr, type, idempotency_key? }` — the field is named `xdr`, not `signedXdr`.
    * Response envelope is `{ success, data: { transactionHash, status }, message }`.
+   *
+   * Idempotency: an idempotency key is auto-generated and sent to the API.
+   * The XDR is also hashed and checked against localStorage to prevent
+   * accidental duplicate submissions on the client side. Records expire
+   * after 24 hours.
    */
   submit: async (
     signedXdr: string,
     type: TransactionType
   ): Promise<SubmittedTransaction> => {
+    const idempotencyKey = idempotencyService.generateKey()
+    const xdrHash = await idempotencyService.hashXdr(signedXdr)
+
+    // Check for duplicate XDR hash (same transaction already submitted)
+    const existingByHash = idempotencyService.findExisting(xdrHash)
+    if (existingByHash) {
+      return {
+        transactionHash: existingByHash.transactionHash,
+        status: 'pending',
+      }
+    }
+
+    // Check for duplicate idempotency key (extremely unlikely but possible
+    // if the same key was reused due to a retry before store() completed)
+    const existingByKey = idempotencyService.findByKey(idempotencyKey)
+    if (existingByKey) {
+      return {
+        transactionHash: existingByKey.transactionHash,
+        status: 'pending',
+      }
+    }
+
     const res = await api.post('/transactions/submit', {
       xdr: signedXdr,
       type,
+      idempotency_key: idempotencyKey,
     })
-    return res.data.data
+
+    const result: SubmittedTransaction = res.data.data
+
+    // Store the record for future deduplication
+    idempotencyService.store({
+      idempotencyKey,
+      xdrHash,
+      transactionHash: result.transactionHash,
+      type,
+      createdAt: new Date().toISOString(),
+    })
+
+    return result
   },
 }
diff --git a/src/types/index.ts b/src/types/index.ts
index 019e8a1..5edf6c2 100644
--- a/src/types/index.ts
+++ b/src/types/index.ts
@@ -98,6 +98,15 @@ export interface SubmittedTransaction {
   status: 'pending'
 }
 
+/** Idempotency record stored in localStorage to prevent duplicate submissions */
+export interface IdempotencyRecord {
+  idempotencyKey: string
+  xdrHash: string
+  transactionHash: string
+  type: TransactionType
+  createdAt: string
+}
+
 export interface PoolInfo {
   totalDeposits: number
   totalLiquidity: number