diff --git a/service/frontend/namespace_handler.go b/service/frontend/namespace_handler.go
index b5a929ff14..a4a82c1718 100644
--- a/service/frontend/namespace_handler.go
+++ b/service/frontend/namespace_handler.go
@@ -4,6 +4,7 @@ package frontend
 
 import (
 	"context"
+	"fmt"
 	"time"
 
 	"github.com/google/uuid"
@@ -946,10 +947,18 @@ func (d *namespaceHandler) upsertCustomSearchAttributesAliases(
 	upsert map[string]string,
 ) (map[string]string, error) {
 	result := util.CloneMapNonNil(current)
+	currentAliasToField := util.InverseMap(current)
 	for key, value := range upsert {
 		if value == "" {
 			delete(result, key)
 		} else if _, ok := current[key]; !ok {
+			if _, aliasExists := currentAliasToField[value]; aliasExists {
+				d.logger.Warn(
+					fmt.Sprintf(errSearchAttributeAlreadyExistsMessage, value),
+					tag.String("visibility-search-attribute", value),
+				)
+				continue
+			}
 			result[key] = value
 		} else {
 			return nil, errCustomSearchAttributeFieldAlreadyAllocated
diff --git a/service/frontend/namespace_handler_test.go b/service/frontend/namespace_handler_test.go
index 590449dadd..5a3aacedd5 100644
--- a/service/frontend/namespace_handler_test.go
+++ b/service/frontend/namespace_handler_test.go
@@ -1955,3 +1955,58 @@ func (s *namespaceHandlerCommonSuite) TestWorkflowRuleEviction() {
 func (s *namespaceHandlerCommonSuite) getRandomNamespace() string {
 	return "namespace" + uuid.NewString()
 }
+
+func (s *namespaceHandlerCommonSuite) TestUpsertCustomSearchAttributesAliases() {
+	tests := []struct {
+		name        string
+		current     map[string]string
+		upsert      map[string]string
+		expected    map[string]string
+		expectedErr error
+	}{
+		{
+			name:     "add to empty",
+			current:  map[string]string{},
+			upsert:   map[string]string{"Keyword01": "MyAttr"},
+			expected: map[string]string{"Keyword01": "MyAttr"},
+		},
+		{
+			name:     "add to existing",
+			current:  map[string]string{"Keyword01": "MyAttr"},
+			upsert:   map[string]string{"Keyword02": "OtherAttr"},
+			expected: map[string]string{"Keyword01": "MyAttr", "Keyword02": "OtherAttr"},
+		},
+		{
+			name:        "field already allocated",
+			current:     map[string]string{"Keyword01": "MyAttr"},
+			upsert:      map[string]string{"Keyword01": "NewAttr"},
+			expectedErr: errCustomSearchAttributeFieldAlreadyAllocated,
+		},
+		{
+			// Race condition: a concurrent request already claimed the same alias for a
+			// different field. The alias already exists so we skip it — idempotent success.
+			name:     "alias already in use by a different field is skipped",
+			current:  map[string]string{"Keyword01": "RaceTestAttr"},
+			upsert:   map[string]string{"Keyword03": "RaceTestAttr"},
+			expected: map[string]string{"Keyword01": "RaceTestAttr"},
+		},
+		{
+			name:     "delete existing",
+			current:  map[string]string{"Keyword01": "MyAttr"},
+			upsert:   map[string]string{"Keyword01": ""},
+			expected: map[string]string{},
+		},
+	}
+
+	for _, tc := range tests {
+		s.Run(tc.name, func() {
+			result, err := s.handler.upsertCustomSearchAttributesAliases(tc.current, tc.upsert)
+			if tc.expectedErr != nil {
+				s.Equal(tc.expectedErr, err)
+			} else {
+				s.Require().NoError(err)
+				s.Equal(tc.expected, result)
+			}
+		})
+	}
+}