diff --git a/contract/passtorage.go b/contract/passtorage.go
index e2c6aae..13f921d 100644
--- a/contract/passtorage.go
+++ b/contract/passtorage.go
@@ -7,8 +7,12 @@ import (
 
 func CreateUser(login string, password interop.Hash256) {
 	ctx := storage.GetContext()
-	// NOTE: maybe we should handle case where user already exists
-	storage.Put(ctx, login, password)
+	storedValue := storage.Get(ctx, login).(interop.Hash256)
+	if storedValue != nil {
+		panic("This user already exists")
+	} else {
+		storage.Put(ctx, login, password)
+	}
 }
 
 func CheckUser(login string, givenPassword interop.Hash256) {