diff --git a/mitmproxy/flow.py b/mitmproxy/flow.py
index 294aba26d..dc778404d 100644
--- a/mitmproxy/flow.py
+++ b/mitmproxy/flow.py
@@ -99,6 +99,7 @@ class Flow(stateobject.StateObject):
         return d
 
     def set_state(self, state):
+        state = state.copy()
         state.pop("version")
         if "backup" in state:
             self._backup = state.pop("backup")
diff --git a/mitmproxy/http.py b/mitmproxy/http.py
index c09778fea..7762647b4 100644
--- a/mitmproxy/http.py
+++ b/mitmproxy/http.py
@@ -56,6 +56,7 @@ class HTTPRequest(http.Request):
         return state
 
     def set_state(self, state):
+        state = state.copy()
         self.is_replay = state.pop("is_replay")
         super().set_state(state)
 
diff --git a/test/mitmproxy/test_flow.py b/test/mitmproxy/test_flow.py
index 7f9d577bc..fcc766b54 100644
--- a/test/mitmproxy/test_flow.py
+++ b/test/mitmproxy/test_flow.py
@@ -84,6 +84,17 @@ class TestSerialize:
         with pytest.raises(Exception, match="version"):
             list(r.stream())
 
+    def test_copy(self):
+        """
+        _backup may be shared across instances. That should not raise errors.
+        """
+        f = tflow.tflow()
+        f.backup()
+        f.request.path = "/foo"
+        f2 = f.copy()
+        f2.revert()
+        f.revert()
+
 
 class TestFlowMaster:
     def test_load_flow_reverse(self):