diff --git a/docs/spec/components/schemas/UserParams.yaml b/docs/spec/components/schemas/UserParams.yaml
index e41ffcf..731b136 100644
--- a/docs/spec/components/schemas/UserParams.yaml
+++ b/docs/spec/components/schemas/UserParams.yaml
@@ -7,10 +7,15 @@ allOf:
     properties:
       attributes:
         required:
+          - nullifier
           - age_lower_bound
           - nationality
           - sex
         properties:
+          nullifier:
+            type: string
+            example: "2fe34ac35a35d0672dd9759aaee5e052d978e41b062c9da33aea9914c3a386a3"
+            description: "User nullifier"
           age_lower_bound:
             type: integer
             example: 18
diff --git a/internal/data/pg/verify_users.go b/internal/data/pg/verify_users.go
index 8af75c6..60b46de 100644
--- a/internal/data/pg/verify_users.go
+++ b/internal/data/pg/verify_users.go
@@ -153,3 +153,8 @@ func (q *VerifyUsersQ) FilterByInternalAID(aid string) data.VerifyUsersQ {
 	q.sel = q.sel.Where(sq.Eq{"anonymous_id": aid})
 	return q
 }
+
+func (q *VerifyUsersQ) FilterByNullifier(nullifier string) data.VerifyUsersQ {
+	q.sel = q.sel.Where(sq.Eq{"nullifier": nullifier})
+	return q
+}
diff --git a/internal/data/verify_users.go b/internal/data/verify_users.go
index 7a2c7f5..fb372d5 100644
--- a/internal/data/verify_users.go
+++ b/internal/data/verify_users.go
@@ -35,4 +35,5 @@ type VerifyUsersQ interface {
 	WhereHashID(userId string) VerifyUsersQ
 	WhereCreatedAtLt(createdAt time.Time) VerifyUsersQ
 	FilterByInternalAID(aid string) VerifyUsersQ
+	FilterByNullifier(nullifier string) VerifyUsersQ
 }
diff --git a/internal/service/handlers/verification_callback.go b/internal/service/handlers/verification_callback.go
index 1c90696..e6fbe15 100644
--- a/internal/service/handlers/verification_callback.go
+++ b/internal/service/handlers/verification_callback.go
@@ -1,6 +1,7 @@
 package handlers
 
 import (
+	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"github.com/ethereum/go-ethereum/log"
@@ -11,6 +12,7 @@ import (
 	zk "github.com/rarimo/zkverifier-kit"
 	"gitlab.com/distributed_lab/ape"
 	"gitlab.com/distributed_lab/ape/problems"
+	"math/big"
 	"net/http"
 	"strconv"
 )
@@ -63,6 +65,16 @@ func VerificationCallback(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	nullifier, ok := new(big.Int).SetString(getter.Get(zk.Nullifier), 10)
+	if !ok {
+		Log(r).Error("failed to parse nullifier")
+		ape.RenderErr(w, problems.BadRequest(err)...)
+		return
+	}
+	var nullifierBytes [32]byte
+	nullifier.FillBytes(nullifierBytes[:])
+	nullifierHex := hex.EncodeToString(nullifierBytes[:])
+
 	userIDHash, err := helpers.ExtractEventData(getter)
 	if err != nil {
 		Log(r).WithError(err).Errorf("failed to extract user hash from event data")
@@ -88,6 +100,22 @@ func VerificationCallback(w http.ResponseWriter, r *http.Request) {
 		eventID = verifiedUser.EventId
 	}
 
+	if verifiedUser.Uniqueness {
+		byNullifier, dbErr := VerifyUsersQ(r).FilterByNullifier(nullifierHex).Get()
+		if dbErr != nil {
+			Log(r).Error("Failed to get user by nullifier")
+			ape.RenderErr(w, problems.BadRequest(dbErr)...)
+			return
+		}
+
+		if byNullifier != nil && byNullifier.UserIDHash != verifiedUser.UserIDHash {
+			Log(r).WithError(err).Errorf("User with this nullifier [%s] but a different userIDHash already exists", nullifierHex)
+			verifiedUser.Status = "failed_verification"
+		} else {
+			verifiedUser.Nullifier = nullifierHex
+		}
+	}
+
 	var verifyOpts = []zk.VerifyOption{
 		zk.WithProofSelectorValue(getter.Get(zk.Selector)),
 		zk.WithAgeAbove(verifiedUser.AgeLowerBound), // if not required -1
diff --git a/internal/service/handlers/verification_callback_light.go b/internal/service/handlers/verification_callback_light.go
index 4313aed..32f73b2 100644
--- a/internal/service/handlers/verification_callback_light.go
+++ b/internal/service/handlers/verification_callback_light.go
@@ -97,13 +97,6 @@ func VerificationSignatureCallback(w http.ResponseWriter, r *http.Request) {
 	anonymousID.FillBytes(anonymousIDBytes[:])
 	anonymousIDHex := hex.EncodeToString(anonymousIDBytes[:])
 
-	byAnonymousID, err := VerifyUsersQ(r).FilterByInternalAID(anonymousIDHex).Get()
-	if err != nil {
-		Log(r).Error("Failed to get user by anonymous_id")
-		ape.RenderErr(w, problems.BadRequest(err)...)
-		return
-	}
-
 	verifiedUser, err := VerifyUsersQ(r).WhereHashID(userIDHash).Get()
 	if err != nil {
 		Log(r).WithError(err).Errorf("failed to get user with userHashID [%s]", userIDHash)
@@ -124,13 +117,43 @@ func VerificationSignatureCallback(w http.ResponseWriter, r *http.Request) {
 	}
 
 	verifiedUser.Status = "verified"
-	if byAnonymousID != nil && byAnonymousID.UserIDHash != verifiedUser.UserIDHash {
-		Log(r).WithError(err).Errorf("User with anonymous_id [%s] but a different userIDHash already exists", anonymousIDHex)
-		verifiedUser.Status = "failed_verification"
-	} else {
-		verifiedUser.Nullifier = nullifierHex
-		verifiedUser.AnonymousID = anonymousIDHex
+
+	if verifiedUser.Uniqueness {
+		byAnonymousID, dbErr := VerifyUsersQ(r).FilterByInternalAID(anonymousIDHex).Get()
+		if dbErr != nil {
+			Log(r).Error("Failed to get user by anonymous_id")
+			ape.RenderErr(w, problems.BadRequest(dbErr)...)
+			return
+		}
+
+		byNullifier, dbErr := VerifyUsersQ(r).FilterByNullifier(nullifierHex).Get()
+		if dbErr != nil {
+			Log(r).Error("Failed to get user by nullifier")
+			ape.RenderErr(w, problems.BadRequest(dbErr)...)
+			return
+		}
+
+		if byAnonymousID != nil {
+			if byAnonymousID.UserIDHash != verifiedUser.UserIDHash {
+				Log(r).WithError(err).Errorf("User with anonymous_id [%s] but a different userIDHash already exists", anonymousIDHex)
+				verifiedUser.Status = "failed_verification"
+				return
+			}
+		} else {
+			verifiedUser.AnonymousID = anonymousIDHex
+		}
+
+		if byNullifier != nil {
+			if byNullifier.UserIDHash != verifiedUser.UserIDHash {
+				Log(r).WithError(err).Errorf("User with nullifier [%s] but a different userIDHash already exists", nullifierHex)
+				verifiedUser.Status = "failed_verification"
+				return
+			}
+		} else {
+			verifiedUser.Nullifier = nullifierHex
+		}
 	}
+
 	if eventData != userIDHash {
 		Log(r).WithError(err).Errorf("failed to verify user: EventData from pub-signals [%s] != userIdHash from db [%s]", eventData, userIDHash)
 		verifiedUser.Status = "failed_verification"
diff --git a/internal/service/responses/get_user.go b/internal/service/responses/get_user.go
index 2c470f2..ac3510c 100644
--- a/internal/service/responses/get_user.go
+++ b/internal/service/responses/get_user.go
@@ -13,6 +13,7 @@ func NewGetUsersByIdResponse(user data.VerifyUsers) resources.UserParamsRequest
 				Type: resources.USER,
 			},
 			Attributes: resources.UserParamsAttributes{
+				Nullifier:     user.Nullifier,
 				AgeLowerBound: int32(user.AgeLowerBound),
 				Nationality:   user.Nationality,
 				Sex:           user.Sex,
diff --git a/resources/model_user_params_attributes.go b/resources/model_user_params_attributes.go
index 09f731a..9b0f87a 100644
--- a/resources/model_user_params_attributes.go
+++ b/resources/model_user_params_attributes.go
@@ -9,6 +9,8 @@ type UserParamsAttributes struct {
 	AgeLowerBound int32 `json:"age_lower_bound"`
 	// User nationality
 	Nationality string `json:"nationality"`
+	// User nullifier
+	Nullifier string `json:"nullifier"`
 	// User sex
 	Sex string `json:"sex"`
 }