diff --git a/pkg/ring/model.go b/pkg/ring/model.go
index 82d0f9ccb3..5a4f134c8a 100644
--- a/pkg/ring/model.go
+++ b/pkg/ring/model.go
@@ -289,7 +289,17 @@ func (d *Desc) mergeWithTime(mergeable memberlist.Mergeable, localCAS bool, now
 	}
 
 	// resolveConflicts allocates lot of memory, so if we can avoid it, do that.
-	if tokensChanged && conflictingTokensExist(thisIngesterMap) {
+	// Check when tokens changed or any ingester is in JOINING state (observe period).
+	shouldCheckConflicts := tokensChanged
+	if !shouldCheckConflicts {
+		for _, ing := range thisIngesterMap {
+			if ing.State == JOINING {
+				shouldCheckConflicts = true
+				break
+			}
+		}
+	}
+	if shouldCheckConflicts && conflictingTokensExist(thisIngesterMap) {
 		resolveConflicts(thisIngesterMap)
 	}
 
@@ -781,23 +791,40 @@ func (d *Desc) FindDifference(o codec.MultiKey) (any, []string, error) {
 			} else if oing.Timestamp == ing.Timestamp && ing.State != LEFT && oing.State == LEFT {
 				// we accept LEFT even if timestamp hasn't changed
 				toUpdated.Ingesters[name] = oing
-				if !tokensEqual(ing.Tokens, oing.Tokens) {
-					tokensChanged = true
-				}
+			}
+		}
+	}
+
+	// Check for token conflicts when tokens changed or when any ingester is still
+	// in JOINING state (observe period). During the observe period, concurrent joins
+	// may have produced duplicate tokens that need resolution.
+	shouldCheckConflicts := tokensChanged
+	if !shouldCheckConflicts {
+		for _, ing := range out.Ingesters {
+			if ing.State == JOINING {
+				shouldCheckConflicts = true
+				break
 			}
 		}
 	}
 
 	// resolveConflicts allocates a lot of memory, so if we can avoid it, do that.
-	if tokensChanged && conflictingTokensExist(out.Ingesters) {
+	if shouldCheckConflicts && conflictingTokensExist(out.Ingesters) {
 		resolveConflicts(out.Ingesters)
 
-		//Recheck if any instance was updated by the resolveConflict
-		//All ingesters in toUpdated have already passed the timestamp check, so we can skip checking again
+		// Refresh all entries already in toUpdated (their tokens may have changed).
 		for name := range toUpdated.Ingesters {
-			//name must appear in out Ingesters, so we can skip the contains key check
 			toUpdated.Ingesters[name] = out.Ingesters[name]
 		}
+		// Also include any existing ingester whose tokens were changed by resolveConflicts.
+		for name, oing := range out.Ingesters {
+			if _, alreadyUpdated := toUpdated.Ingesters[name]; alreadyUpdated {
+				continue
+			}
+			if ing, ok := d.Ingesters[name]; ok && !tokensEqual(ing.Tokens, oing.Tokens) {
+				toUpdated.Ingesters[name] = oing
+			}
+		}
 	}
 
 	return toUpdated, toDelete, nil
diff --git a/pkg/ring/model_test.go b/pkg/ring/model_test.go
index 16295ff354..2c71474778 100644
--- a/pkg/ring/model_test.go
+++ b/pkg/ring/model_test.go
@@ -5,6 +5,7 @@ import (
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestInstanceDesc_IsHealthy_ForIngesterOperations(t *testing.T) {
@@ -746,6 +747,50 @@ func TestDesc_FindDifference(t *testing.T) {
 	}
 }
 
+func TestDesc_FindDifference_ConcurrentConflictResolutionIsDeterministic(t *testing.T) {
+	// Simulate two ingesters with duplicate tokens both doing CAS at the same time
+	// during the observe period (JOINING state). Both read the same DDB state (current)
+	// and produce the same new state (out). FindDifference must produce identical
+	// toUpdate results so they don't write conflicting resolutions to DDB.
+	current := &Desc{Ingesters: map[string]InstanceDesc{
+		"ing-A": {Addr: "addr-A", Tokens: []uint32{1, 2, 3, 10, 20}, Timestamp: 100, State: JOINING},
+		"ing-B": {Addr: "addr-B", Tokens: []uint32{1, 2, 3, 30, 40}, Timestamp: 100, State: JOINING},
+	}}
+
+	// ing-A does a heartbeat CAS (only timestamp changes)
+	outA := &Desc{Ingesters: map[string]InstanceDesc{
+		"ing-A": {Addr: "addr-A", Tokens: []uint32{1, 2, 3, 10, 20}, Timestamp: 110, State: JOINING},
+		"ing-B": {Addr: "addr-B", Tokens: []uint32{1, 2, 3, 30, 40}, Timestamp: 100, State: JOINING},
+	}}
+
+	// ing-B does a heartbeat CAS (only timestamp changes)
+	outB := &Desc{Ingesters: map[string]InstanceDesc{
+		"ing-A": {Addr: "addr-A", Tokens: []uint32{1, 2, 3, 10, 20}, Timestamp: 100, State: JOINING},
+		"ing-B": {Addr: "addr-B", Tokens: []uint32{1, 2, 3, 30, 40}, Timestamp: 110, State: JOINING},
+	}}
+
+	toUpdateA, _, errA := current.FindDifference(outA)
+	toUpdateB, _, errB := current.FindDifference(outB)
+
+	require.NoError(t, errA)
+	require.NoError(t, errB)
+
+	// Both must resolve the conflict: ing-A wins tokens 1,2,3 (lower name), ing-B loses them.
+	updatedA := toUpdateA.(*Desc)
+	updatedB := toUpdateB.(*Desc)
+
+	// ing-A's resolution should strip tokens 1,2,3 from ing-B
+	require.Contains(t, updatedA.Ingesters, "ing-B")
+	assert.Equal(t, []uint32{30, 40}, updatedA.Ingesters["ing-B"].Tokens)
+
+	// ing-B's resolution should also strip tokens 1,2,3 from ing-B
+	require.Contains(t, updatedB.Ingesters, "ing-B")
+	assert.Equal(t, []uint32{30, 40}, updatedB.Ingesters["ing-B"].Tokens)
+
+	// Both produce the same token assignment for ing-B — deterministic resolution.
+	assert.Equal(t, updatedA.Ingesters["ing-B"].Tokens, updatedB.Ingesters["ing-B"].Tokens)
+}
+
 func Test_resolveConflicts(t *testing.T) {
 	tests := []struct {
 		name string