diff --git a/components/net/sf/briar/db/JdbcDatabase.java b/components/net/sf/briar/db/JdbcDatabase.java
index c09508ad2..5363496f6 100644
--- a/components/net/sf/briar/db/JdbcDatabase.java
+++ b/components/net/sf/briar/db/JdbcDatabase.java
@@ -383,8 +383,24 @@ abstract class JdbcDatabase implements Database<Connection> {
 	public void addBatchToAck(Connection txn, ContactId c, BatchId b)
 	throws DbException {
 		PreparedStatement ps = null;
+		ResultSet rs = null;
 		try {
-			String sql = "INSERT INTO batchesToAck (batchId, contactId)"
+			String sql = "SELECT COUNT(batchId) FROM batchesToAck"
+				+ " WHERE batchId = ? AND contactId = ?";
+			ps = txn.prepareStatement(sql);
+			ps.setBytes(1, b.getBytes());
+			ps.setInt(2, c.getInt());
+			rs = ps.executeQuery();
+			boolean found = rs.next();
+			assert found;
+			int count = rs.getInt(1);
+			assert count <= 1;
+			boolean more = rs.next();
+			assert !more;
+			rs.close();
+			ps.close();
+			if(count == 1) return;
+			sql = "INSERT INTO batchesToAck (batchId, contactId)"
 				+ " VALUES (?, ?)";
 			ps = txn.prepareStatement(sql);
 			ps.setBytes(1, b.getBytes());
@@ -393,6 +409,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 			assert rowsAffected == 1;
 			ps.close();
 		} catch(SQLException e) {
+			tryToClose(rs);
 			tryToClose(ps);
 			tryToClose(txn);
 			throw new DbException(e);
@@ -1249,7 +1266,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 			ps.setInt(5, 1);
 			rs = ps.executeQuery();
 			boolean found = rs.next();
-			assert !rs.next();
+			boolean more = rs.next();
+			assert !more;
 			rs.close();
 			ps.close();
 			return found;
diff --git a/test/net/sf/briar/db/H2DatabaseTest.java b/test/net/sf/briar/db/H2DatabaseTest.java
index c66258ad9..9f56e9303 100644
--- a/test/net/sf/briar/db/H2DatabaseTest.java
+++ b/test/net/sf/briar/db/H2DatabaseTest.java
@@ -404,6 +404,33 @@ public class H2DatabaseTest extends TestCase {
 		db.close();
 	}
 
+	@Test
+	public void testDuplicateBatchesReceived() throws DbException {
+		Database<Connection> db = open(false);
+		Connection txn = db.startTransaction();
+
+		// Add a contact and receive the same batch twice
+		assertEquals(contactId, db.addContact(txn, null));
+		db.addBatchToAck(txn, contactId, batchId);
+		db.addBatchToAck(txn, contactId, batchId);
+		db.commitTransaction(txn);
+
+		// The batch ID should only be returned once
+		Collection<BatchId> acks = db.getBatchesToAck(txn, contactId);
+		assertEquals(1, acks.size());
+		assertTrue(acks.contains(batchId));
+
+		// Remove the batch ID
+		db.removeBatchesToAck(txn, contactId, acks);
+
+		// The batch ID should have been removed
+		acks = db.getBatchesToAck(txn, contactId);
+		assertEquals(0, acks.size());
+
+		db.commitTransaction(txn);
+		db.close();
+	}
+
 	@Test
 	public void testRemoveAckedBatch() throws DbException {
 		Database<Connection> db = open(false);