diff --git a/web/server.go b/web/server.go
index 9a151e6..0cd2f9a 100644
--- a/web/server.go
+++ b/web/server.go
@@ -604,7 +604,21 @@ func (s *Server) handleCreateRacer() http.HandlerFunc {
 			return
 		}
 
-		// Create racer
+		// Check if car number is unique before creating
+		isUnique, err := s.db.IsCarNumberUnique(carNumber)
+		if err != nil {
+			s.logger.Error("Failed to check car number uniqueness", "error", err)
+			w.Header().Set("Content-Type", "text/html")
+			w.Write([]byte(`<div class="alert alert-danger">Failed to validate car number</div>`))
+			return
+		}
+
+		if !isUnique {
+			w.Header().Set("Content-Type", "text/html")
+			w.Write([]byte(`<div class="alert alert-danger">Car number is already in use</div>`))
+			return
+		}
+
 		id, err := s.db.CreateRacer(firstName, lastName, carNumber, carWeight, groupID)
 		if err != nil {
 			s.logger.Error("Failed to create racer", "error", err)