Validate allow_duplicate with prevent_initial_call.

dash/_callback.py

@@ -241,13 +241,19 @@ def insert_callback(
     if prevent_initial_call is None:
         prevent_initial_call = config_prevent_initial_callbacks
 
+    _validate.validate_duplicate_output(
+        output, prevent_initial_call, config_prevent_initial_callbacks
+    )
+
     callback_id = create_callback_id(output)
     callback_spec = {
         "output": callback_id,
         "inputs": [c.to_dict() for c in inputs],
         "state": [c.to_dict() for c in state],
         "clientside_function": None,
-        "prevent_initial_call": prevent_initial_call,
+        # prevent_initial_call can be a string "initial_duplicates"
+        # which should not prevent the initial call.
+        "prevent_initial_call": prevent_initial_call is True,
         "long": long
         and {
             "interval": long["interval"],

dash/_validate.py

@@ -521,3 +521,30 @@ def validate_long_callbacks(callback_map):
                 f"Long callback circular error!\n{circular} is used as input for a long callback"
                 f" but also used as output from an input that is updated with progress or running argument."
             )
+
+
+def validate_duplicate_output(
+    output, prevent_initial_call, config_prevent_initial_call
+):
+
+    if "initial_duplicate" in (prevent_initial_call, config_prevent_initial_call):
+        return
+
+    def _valid(out):
+        if out.allow_duplicate is True and not (
+            prevent_initial_call or config_prevent_initial_call
+        ):
+            raise exceptions.DuplicateCallback(
+                "allow_duplicate requires prevent_initial_call to be True. The order of the call is not"
+                " guaranteed to be the same on every page load. "
+                "To enable duplicate callback with initial call, set prevent_initial_call='initial_duplicate' "
+                " or globally in the config prevent_initial_callbacks='initial_duplicate'"
+            )
+
+    if isinstance(output, (list, tuple)):
+        for o in output:
+            _valid(o)
+
+        return
+
+    _valid(output)

tests/unit/library/test_validate.py

@@ -2,8 +2,8 @@
 
 from dash import Output
 from dash.html import Div
-from dash.exceptions import InvalidCallbackReturnValue
-from dash._validate import fail_callback_output
+from dash.exceptions import InvalidCallbackReturnValue, DuplicateCallback
+from dash._validate import fail_callback_output, validate_duplicate_output
 
 
 @pytest.mark.parametrize(
@@ -36,3 +36,34 @@ def test_ddvl001_fail_handler_fails_correctly(val):
 
     with pytest.raises(InvalidCallbackReturnValue):
         fail_callback_output(val, outputs)
+
+
+@pytest.mark.parametrize(
+    "output, prevent_initial_call, config_prevent_initial_call, expect_error",
+    [
+        (Output("a", "a", allow_duplicate=True), True, False, False),
+        (Output("a", "a", allow_duplicate=True), False, True, False),
+        (Output("a", "a", allow_duplicate=True), True, True, False),
+        (Output("a", "a", allow_duplicate=True), False, False, True),
+        (Output("a", "a", allow_duplicate=True), "initial_duplicates", False, False),
+        (Output("a", "a", allow_duplicate=True), False, "initial_duplicates", False),
+        (Output("a", "a"), False, False, False),
+    ],
+)
+def test_ddv002_allow_duplicate_validation(
+    output, prevent_initial_call, config_prevent_initial_call, expect_error
+):
+    valid = False
+    if expect_error:
+        with pytest.raises(DuplicateCallback) as err:
+            validate_duplicate_output(
+                output, prevent_initial_call, config_prevent_initial_call
+            )
+        valid = err is not None
+    else:
+        validate_duplicate_output(
+            output, prevent_initial_call, config_prevent_initial_call
+        )
+        valid = True
+
+    assert valid