diff --git a/router/java/src/net/i2p/router/transport/udp/OutboundMessageFragments.java b/router/java/src/net/i2p/router/transport/udp/OutboundMessageFragments.java
index 4f2c9a2d8..10e798999 100644
--- a/router/java/src/net/i2p/router/transport/udp/OutboundMessageFragments.java
+++ b/router/java/src/net/i2p/router/transport/udp/OutboundMessageFragments.java
@@ -4,6 +4,7 @@ import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.Comparator;
+import java.util.HashSet;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Set;
@@ -386,7 +387,7 @@ class OutboundMessageFragments {
         int piggybackedPartialACK = partialACKBitfields.size();
         // getCurrentFullACKs() already makes a copy, do we need to copy again?
         // YES because buildPacket() now removes them (maybe)
-        List<Long> remaining = new ArrayList<Long>(msgIds);
+        Set<Long> remaining = new HashSet<Long>(msgIds);
 
         // build the list of fragments to send
         List<Fragment> toSend = new ArrayList<Fragment>(8);
diff --git a/router/java/src/net/i2p/router/transport/udp/PacketBuilder.java b/router/java/src/net/i2p/router/transport/udp/PacketBuilder.java
index efa408187..11def9b7f 100644
--- a/router/java/src/net/i2p/router/transport/udp/PacketBuilder.java
+++ b/router/java/src/net/i2p/router/transport/udp/PacketBuilder.java
@@ -5,6 +5,7 @@ import java.net.InetAddress;
 import java.net.UnknownHostException;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
@@ -290,7 +291,7 @@ class PacketBuilder {
      * @return null on error
      */
     public UDPPacket buildPacket(OutboundMessageState state, int fragment, PeerState peer,
-                                 List<Long> ackIdsRemaining, int newAckCount,
+                                 Collection<Long> ackIdsRemaining, int newAckCount,
                                  List<ACKBitfield> partialACKsRemaining) {
         List<Fragment> frags = Collections.singletonList(new Fragment(state, fragment));
         return buildPacket(frags, peer, ackIdsRemaining, newAckCount, partialACKsRemaining);
@@ -302,7 +303,7 @@ class PacketBuilder {
      *  @since 0.9.16
      */
     public UDPPacket buildPacket(List<Fragment> fragments, PeerState peer,
-                                 List<Long> ackIdsRemaining, int newAckCount,
+                                 Collection<Long> ackIdsRemaining, int newAckCount,
                                  List<ACKBitfield> partialACKsRemaining) {
         StringBuilder msg = null;
         if (_log.shouldLog(Log.INFO)) {