only mark backend vars as dirty if they have changed (#4494)

reflex/state.py

@@ -1288,6 +1288,9 @@ class BaseState(Base, ABC, extra=pydantic.Extra.allow):
             return
 
         if name in self.backend_vars:
+            # abort if unchanged
+            if self._backend_vars.get(name) == value:
+                return
             self._backend_vars.__setitem__(name, value)
             self.dirty_vars.add(name)
             self._mark_dirty()