diff --git a/federationsender/internal/perform.go b/federationsender/internal/perform.go
index 30950f528..f9c295644 100644
--- a/federationsender/internal/perform.go
+++ b/federationsender/internal/perform.go
@@ -193,19 +193,6 @@ func (r *FederationSenderInternalAPI) performJoinUsingServer(
 		return fmt.Errorf("joinCtx.CheckSendJoinResponse: %w", err)
 	}
 
-	// It's possible that the remote server has included our new
-	// membership event in the room state in the send_join response,
-	// but if that's the case, then we'll get a duplicate state block
-	// error if we try to send that along with our own copy of the
-	// event. The simple way around this is just to prune the event
-	// from the state if we find it.
-	for i, ev := range respState.StateEvents {
-		if ev.Type() == gomatrixserverlib.MRoomMember && ev.StateKeyEquals(userID) {
-			respState.StateEvents = append(respState.StateEvents[i:], respState.StateEvents[:i+1]...)
-			break
-		}
-	}
-
 	// If we successfully performed a send_join above then the other
 	// server now thinks we're a part of the room. Send the newly
 	// returned state to the roomserver to update our local view.
diff --git a/roomserver/internal/helpers/auth.go b/roomserver/internal/helpers/auth.go
index 060f0a0e9..524a54510 100644
--- a/roomserver/internal/helpers/auth.go
+++ b/roomserver/internal/helpers/auth.go
@@ -36,7 +36,7 @@ func CheckAuthEvents(
 	if err != nil {
 		return nil, err
 	}
-	// TODO: check for duplicate state keys here.
+	authStateEntries = types.DeduplicateStateEntries(authStateEntries)
 
 	// Work out which of the state events we actually need.
 	stateNeeded := gomatrixserverlib.StateNeededForAuth([]gomatrixserverlib.Event{event.Unwrap()})
diff --git a/roomserver/internal/input/input_events.go b/roomserver/internal/input/input_events.go
index 19089cbd3..ec429b2dd 100644
--- a/roomserver/internal/input/input_events.go
+++ b/roomserver/internal/input/input_events.go
@@ -170,6 +170,7 @@ func (r *Inputer) calculateAndSetState(
 		if entries, err = r.DB.StateEntriesForEventIDs(ctx, input.StateEventIDs); err != nil {
 			return fmt.Errorf("r.DB.StateEntriesForEventIDs: %w", err)
 		}
+		entries = types.DeduplicateStateEntries(entries)
 
 		if stateAtEvent.BeforeStateSnapshotNID, err = r.DB.AddState(ctx, roomInfo.RoomNID, nil, entries); err != nil {
 			return fmt.Errorf("r.DB.AddState: %w", err)
diff --git a/roomserver/types/types.go b/roomserver/types/types.go
index 60f4b0fd5..f573ce3e0 100644
--- a/roomserver/types/types.go
+++ b/roomserver/types/types.go
@@ -16,6 +16,8 @@
 package types
 
 import (
+	"sort"
+
 	"github.com/matrix-org/gomatrixserverlib"
 )
 
@@ -72,6 +74,24 @@ func (a StateEntry) LessThan(b StateEntry) bool {
 	return a.EventNID < b.EventNID
 }
 
+// Deduplicate ensures that the latest NIDs are always presented in the case of duplicates.
+func DeduplicateStateEntries(a []StateEntry) []StateEntry {
+	result := a
+	if len(a) < 2 {
+		return a
+	}
+	sort.SliceStable(a, func(i, j int) bool {
+		return a[i].LessThan(a[j])
+	})
+	for i := 0; i < len(result)-1; i++ {
+		if result[i].StateKeyTuple == result[i+1].StateKeyTuple {
+			result = append(result[:i], result[i+1:]...)
+			i--
+		}
+	}
+	return result
+}
+
 // StateAtEvent is the state before and after a matrix event.
 type StateAtEvent struct {
 	// Should this state overwrite the latest events and memberships of the room?
diff --git a/roomserver/types/types_test.go b/roomserver/types/types_test.go
new file mode 100644
index 000000000..b1e84b821
--- /dev/null
+++ b/roomserver/types/types_test.go
@@ -0,0 +1,26 @@
+package types
+
+import (
+	"testing"
+)
+
+func TestDeduplicateStateEntries(t *testing.T) {
+	entries := []StateEntry{
+		{StateKeyTuple{1, 1}, 1},
+		{StateKeyTuple{1, 1}, 2},
+		{StateKeyTuple{1, 1}, 3},
+		{StateKeyTuple{2, 2}, 4},
+		{StateKeyTuple{2, 3}, 5},
+		{StateKeyTuple{3, 3}, 6},
+	}
+	expected := []EventNID{3, 4, 5, 6}
+	entries = DeduplicateStateEntries(entries)
+	if len(entries) != 4 {
+		t.Fatalf("Expected 4 entries, got %d entries", len(entries))
+	}
+	for i, v := range entries {
+		if v.EventNID != expected[i] {
+			t.Fatalf("Expected position %d to be %d but got %d", i, expected[i], v.EventNID)
+		}
+	}
+}