[dask] raise more informative error for duplicates in 'machines' (fixes

#4057) (#4059)

* [dask] raise more informative error for duplicates in 'machines'

* uncomment

* avoid test failure

* Revert "avoid test failure"

This reverts commit 9442bdf.

python-package/lightgbm/dask.py

@@ -153,6 +153,10 @@ def _machines_to_worker_map(machines: str, worker_addresses: List[str]) -> Dict[
         Dictionary where keys are work addresses in the form expected by Dask and values are a port for LightGBM to use.
     """
     machine_addresses = machines.split(",")
+
+    if len(set(machine_addresses)) != len(machine_addresses):
+        raise ValueError(f"Found duplicates in 'machines' ({machines}). Each entry in 'machines' must be a unique IP-port combination.")
+
     machine_to_port = defaultdict(set)
     for address in machine_addresses:
         host, port = address.split(":")

tests/python_package_test/test_dask.py

@@ -1116,6 +1116,7 @@ def test_machines_should_be_used_if_provided(task, output):
             client.rebalance()
 
         n_workers = len(client.scheduler_info()['workers'])
+        assert n_workers > 1
         open_ports = [lgb.dask._find_random_open_port() for _ in range(n_workers)]
         dask_model = dask_model_factory(
             n_estimators=5,
@@ -1134,6 +1135,17 @@ def test_machines_should_be_used_if_provided(task, output):
                 s.bind(('127.0.0.1', open_ports[0]))
                 dask_model.fit(dX, dy, group=dg)
 
+        # an informative error should be raised if "machines" has duplicates
+        one_open_port = lgb.dask._find_random_open_port()
+        dask_model.set_params(
+            machines=",".join([
+                "127.0.0.1:" + str(one_open_port)
+                for _ in range(n_workers)
+            ])
+        )
+        with pytest.raises(ValueError, match="Found duplicates in 'machines'"):
+            dask_model.fit(dX, dy, group=dg)
+
 
 @pytest.mark.parametrize(
     "classes",