Fix MetricCollection when input are metrics that return dicts with same keywords

CHANGELOG.md

@@ -32,6 +32,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed support for pixelwise MSE ([#2017](https://github.com/Lightning-AI/torchmetrics/pull/2017)
 
 
+- Fixed bug in `MetricCollection` when used with multiple metrics that return dicts with same keys ([#2027](https://github.com/Lightning-AI/torchmetrics/pull/2027)
+
+
 ## [1.1.0] - 2023-08-22
 
 ### Added

src/torchmetrics/collections.py

@@ -337,6 +337,8 @@ def _compute_and_reduce(
 
             if isinstance(res, dict):
                 for key, v in res.items():
+                    stripped_k = k.replace(m.prefix, "").replace(m.postfix, "")
+                    key = f"{stripped_k}_{key}"
                     if hasattr(m, "prefix") and m.prefix is not None:
                         key = f"{m.prefix}{key}"
                     if hasattr(m, "postfix") and m.postfix is not None:

tests/unittests/bases/test_collections.py

@@ -614,11 +614,33 @@ def test_nested_collections(input_collections):
     assert "valmetrics/micro_MulticlassPrecision" in val
 
 
-def test_double_nested_collections():
+@pytest.mark.parametrize(
+    ("base_metrics", "expected"),
+    [
+        (
+            DummyMetricMultiOutputDict(),
+            (
+                "prefix2_prefix1_DummyMetricMultiOutputDict_output1_postfix1_postfix2",
+                "prefix2_prefix1_DummyMetricMultiOutputDict_output2_postfix1_postfix2",
+            ),
+        ),
+        (
+            {"metric1": DummyMetricMultiOutputDict(), "metric2": DummyMetricMultiOutputDict()},
+            (
+                "prefix2_prefix1_metric1_output1_postfix1_postfix2",
+                "prefix2_prefix1_metric1_output2_postfix1_postfix2",
+                "prefix2_prefix1_metric2_output1_postfix1_postfix2",
+                "prefix2_prefix1_metric2_output2_postfix1_postfix2",
+            ),
+        ),
+    ],
+)
+def test_double_nested_collections(base_metrics, expected):
     """Test that double nested collections gets flattened to a single collection."""
-    collection1 = MetricCollection([DummyMetricMultiOutputDict()], prefix="prefix1_", postfix="_postfix1")
+    collection1 = MetricCollection(base_metrics, prefix="prefix1_", postfix="_postfix1")
     collection2 = MetricCollection([collection1], prefix="prefix2_", postfix="_postfix2")
     x = torch.randn(10).sum()
     val = collection2(x)
-    assert "prefix2_prefix1_output1_postfix1_postfix2" in val
-    assert "prefix2_prefix1_output2_postfix1_postfix2" in val
+
+    for key in val:
+        assert key in expected