diff --git a/app/contract/contracts/Folder/src/escrow.rs b/app/contract/contracts/Folder/src/escrow.rs
index 9c5bd7aa8..a01d044af 100644
--- a/app/contract/contracts/Folder/src/escrow.rs
+++ b/app/contract/contracts/Folder/src/escrow.rs
@@ -352,11 +352,40 @@ pub fn deposit_partial(
     // INV-3: validated, overflow-safe expiry computation
     let expires_at = compute_expires_at(env, timeout_secs)?;
 
-    let commitment = commitment::create_amount_commitment(env, owner.clone(), amount_due, salt)?;
-    let now = env.ledger().timestamp();
+    // Derive a deterministic escrow_id that includes initial_payment so
+    // identical retries are detected and idempotently return the existing
+    // commitment without creating a duplicate entry.
+    let escrow_id = escrow_id::derive_partial_escrow_id(
+        env,
+        &token,
+        amount_due,
+        initial_payment,
+        &owner,
+        &salt,
+        timeout_secs,
+        &arbiter,
+    )?;
+    if let Some(existing) = get_escrow_id_mapping(env, &escrow_id) {
+        return Ok(existing);
+    }
 
-    let token_client = token::Client::new(env, &token);
+    let (commitment, legacy_commitment) =
+        commitment::amount_commitment_hashes(env, &owner, amount_due, &salt)?;
     let commitment_bytes: Bytes = commitment.clone().into();
+
+    // Reject duplicate commitment to prevent overwriting an existing escrow.
+    if has_escrow(env, &commitment_bytes) {
+        return Err( RustAcademyError::CommitmentAlreadyExists);
+    }
+    if legacy_commitment != commitment {
+        let legacy_bytes: Bytes = legacy_commitment.into();
+        if has_escrow(env, &legacy_bytes) {
+            return Err( RustAcademyError::CommitmentAlreadyExists);
+        }
+    }
+
+    let now = env.ledger().timestamp();
+    let token_client = token::Client::new(env, &token);
     let entry = EscrowEntry {
         token, // moved
         amount_due,
@@ -371,6 +400,12 @@ pub fn deposit_partial(
     };
 
     put_escrow(env, &commitment_bytes, &entry);
+    // Store forward (escrow_id → commitment) and reverse (commitment → escrow_id)
+    // mappings so the backend indexer can correlate partial escrow events with
+    // their stable IDs, and terminal cleanup can drop the dedup mapping.
+    put_escrow_id_mapping(env, &escrow_id, &commitment);
+    put_commitment_escrow_id(env, &commitment_bytes, &escrow_id);
+
     token_client.transfer(&owner, env.current_contract_address(), &initial_payment);
 
     let token_addr = token_client.address.clone();
diff --git a/app/contract/contracts/Folder/src/escrow_id.rs b/app/contract/contracts/Folder/src/escrow_id.rs
index a14797fbc..49a7c1a27 100644
--- a/app/contract/contracts/Folder/src/escrow_id.rs
+++ b/app/contract/contracts/Folder/src/escrow_id.rs
@@ -135,3 +135,67 @@ pub fn derive_escrow_id(
 
     Ok(env.crypto().sha256(&payload).into())
 }
+
+/// Domain separation tag for partial-escrow-id derivation.
+///
+/// Distinct from [`ESCROW_ID_DOMAIN_TAG`] to prevent cross-protocol collisions
+/// between full-payment and partial-payment escrow IDs.
+pub const PARTIAL_ESCROW_ID_DOMAIN_TAG: &[u8] = b" RustAcademy::PARTIAL_ESCROW_ID::v1";
+
+/// Derive a deterministic 32-byte escrow id for a partial-payment deposit.
+///
+/// Extends the full-payment derivation by committing to both `amount_due` and
+/// `initial_payment`, so two partial escrows with the same `amount_due` but
+/// different initial payments are treated as distinct and never alias each other.
+///
+/// # Errors
+///
+/// - [` RustAcademyError::InvalidAmount`] if `amount_due < 0` or `initial_payment < 0`.
+/// - [` RustAcademyError::InvalidSalt`] if `salt.len() > 1024`.
+pub fn derive_partial_escrow_id(
+    env: &Env,
+    token: &Address,
+    amount_due: i128,
+    initial_payment: i128,
+    owner: &Address,
+    salt: &Bytes,
+    timeout_secs: u64,
+    arbiter: &Option<Address>,
+) -> Result<BytesN<32>,  RustAcademyError> {
+    if amount_due < 0 || initial_payment < 0 {
+        return Err( RustAcademyError::InvalidAmount);
+    }
+    if salt.len() > MAX_SALT_LEN {
+        return Err( RustAcademyError::InvalidSalt);
+    }
+
+    let mut payload = Bytes::new(env);
+
+    payload.append(&Bytes::from_slice(env, PARTIAL_ESCROW_ID_DOMAIN_TAG));
+
+    let token_xdr = token.to_xdr(env);
+    append_len_prefixed(env, &mut payload, &token_xdr);
+
+    payload.append(&Bytes::from_array(env, &amount_due.to_be_bytes()));
+    payload.append(&Bytes::from_array(env, &initial_payment.to_be_bytes()));
+
+    let owner_xdr = owner.to_xdr(env);
+    append_len_prefixed(env, &mut payload, &owner_xdr);
+
+    payload.append(&Bytes::from_array(env, &timeout_secs.to_be_bytes()));
+
+    match arbiter {
+        None => {
+            payload.append(&Bytes::from_array(env, &[ARBITER_TAG_NONE]));
+        }
+        Some(arb) => {
+            payload.append(&Bytes::from_array(env, &[ARBITER_TAG_SOME]));
+            let arb_xdr = arb.to_xdr(env);
+            append_len_prefixed(env, &mut payload, &arb_xdr);
+        }
+    }
+
+    append_len_prefixed(env, &mut payload, salt);
+
+    Ok(env.crypto().sha256(&payload).into())
+}
diff --git a/app/contract/contracts/Folder/src/test.rs b/app/contract/contracts/Folder/src/test.rs
index 4a5e50894..f79a430f8 100644
--- a/app/contract/contracts/Folder/src/test.rs
+++ b/app/contract/contracts/Folder/src/test.rs
@@ -3784,3 +3784,78 @@ fn test_single_arbiter_still_works() {
     );
     assert_eq!(token_client.balance(&recipient), amount);
 }
+
+// ============================================================================
+// deposit_partial duplicate detection and escrow ID mapping (Issue #13)
+// ============================================================================
+
+#[test]
+fn test_deposit_partial_identical_retry_returns_existing_commitment() {
+    let (env, client) = setup();
+    let token = create_test_token(&env);
+    let owner = Address::generate(&env);
+    let amount_due: i128 = 1000;
+    let initial: i128 = 300;
+    let salt = Bytes::from_slice(&env, b"idempotent_salt_partial");
+
+    let token_client = token::StellarAssetClient::new(&env, &token);
+    token_client.mint(&owner, &(initial * 2));
+
+    let c1 = client.deposit_partial(&token, &amount_due, &initial, &owner, &salt, &0, &None);
+    // Identical retry must return the same commitment, not create a duplicate.
+    let c2 = client.deposit_partial(&token, &amount_due, &initial, &owner, &salt, &0, &None);
+    assert_eq!(c1, c2);
+}
+
+#[test]
+fn test_deposit_partial_duplicate_commitment_rejected() {
+    // Two partial deposits with the same (amount_due, salt) must not overwrite each other.
+    let (env, client) = setup();
+    let token = create_test_token(&env);
+    let owner = Address::generate(&env);
+    let amount_due: i128 = 1000;
+    let salt = Bytes::from_slice(&env, b"dup_check_salt");
+
+    let token_client = token::StellarAssetClient::new(&env, &token);
+    token_client.mint(&owner, &2000);
+
+    client.deposit_partial(&token, &amount_due, &200, &owner, &salt, &0, &None);
+    // Changing initial_payment but same (amount_due, salt) key → CommitmentAlreadyExists.
+    let result = client.try_deposit_partial(&token, &amount_due, &300, &owner, &salt, &0, &None);
+    assert!(result.is_err());
+    assert_eq!(
+        result.unwrap_err().unwrap(),
+        crate::errors:: RustAcademyError::CommitmentAlreadyExists
+    );
+}
+
+#[test]
+fn test_deposit_partial_has_escrow_id_mapping() {
+    let (env, client) = setup();
+    let token = create_test_token(&env);
+    let owner = Address::generate(&env);
+    let amount_due: i128 = 2000;
+    let initial: i128 = 500;
+    let salt = Bytes::from_slice(&env, b"id_map_salt");
+
+    let token_client = token::StellarAssetClient::new(&env, &token);
+    token_client.mint(&owner, &initial);
+
+    let commitment = client.deposit_partial(&token, &amount_due, &initial, &owner, &salt, &0, &None);
+
+    // Backend / indexer can correlate the commitment to a stable escrow ID.
+    let escrow_id = crate::escrow_id::derive_partial_escrow_id(
+        &env,
+        &token,
+        amount_due,
+        initial,
+        &owner,
+        &Bytes::from_slice(&env, b"id_map_salt"),
+        0,
+        &None,
+    )
+    .unwrap();
+
+    let mapped = client.get_escrow_id_commitment(&escrow_id);
+    assert_eq!(mapped, Some(commitment));
+}