diff --git a/improver/categorical/utilities.py b/improver/categorical/utilities.py
index 51ba0dc896..6505345dd3 100644
--- a/improver/categorical/utilities.py
+++ b/improver/categorical/utilities.py
@@ -388,6 +388,16 @@ def check_tree(
 
     all_key_words = REQUIRED_KEY_WORDS + OPTIONAL_KEY_WORDS
     all_leaf_key_words = LEAF_REQUIRED_KEY_WORDS + LEAF_OPTIONAL_KEY_WORDS
+
+    # Check that all leaves have a unique "leaf" value
+    all_leaves = [v["leaf"] for v in decision_tree.values() if "leaf" in v.keys()]
+    unique_leaves = set()
+    duplicates = [x for x in all_leaves if x in unique_leaves or unique_leaves.add(x)]
+    if duplicates:
+        issues.append(
+            f"These leaf categories are used more than once: {sorted(list(set(duplicates)))}"
+        )
+
     for node, items in decision_tree.items():
         if "leaf" in items.keys():
             # Check the leaf only contains expected keys
@@ -408,7 +418,8 @@ def check_tree(
             if not isinstance(leaf_target, int):
                 issues.append(f"Leaf '{node}' has non-int target: {leaf_target}")
 
-            # If leaf has "if_night", check it points to another leaf.
+            # If leaf has "if_night", check it points to another leaf
+            # AND that the other leaf does NOT have "if_night".
             if "if_night" in items.keys():
                 target = decision_tree.get(items["if_night"], None)
                 if not target:
@@ -419,6 +430,11 @@ def check_tree(
                     issues.append(
                         f"Target '{items['if_night']}' of leaf '{node}' is not a leaf."
                     )
+                elif "if_night" in target.keys():
+                    issues.append(
+                        f"Night target '{items['if_night']}' of leaf '{node}' "
+                        "also has a night target."
+                    )
             # If leaf has "group", check the group contains at least two members.
             if "group" in items.keys():
                 members = [
diff --git a/improver_tests/categorical/test_utilities.py b/improver_tests/categorical/test_utilities.py
index 04c6482b8c..8116a6e90b 100644
--- a/improver_tests/categorical/test_utilities.py
+++ b/improver_tests/categorical/test_utilities.py
@@ -530,18 +530,6 @@ def modify_tree_fixture(node, key, value):
             ),
         ),
         ("Thunder", "leaf", 10.2, "Leaf 'Thunder' has non-int target: 10.2",),
-        (
-            "Clear_Night",
-            "if_night",
-            "kittens",
-            "Leaf 'Clear_Night' does not point to a valid target (kittens).",
-        ),
-        (
-            "Clear_Night",
-            "if_night",
-            "lightning_shower",
-            "Target 'lightning_shower' of leaf 'Clear_Night' is not a leaf.",
-        ),
         (
             "Clear_Night",
             "pets",
@@ -644,6 +632,52 @@ def test_check_tree(modify_tree, expected):
     assert result == expected
 
 
+@pytest.mark.parametrize("node, key", (("Sunny_Day", "if_night"),))
+@pytest.mark.parametrize(
+    "value, expected",
+    (
+        ("kittens", "Leaf 'Sunny_Day' does not point to a valid target (kittens).",),
+        (
+            "Partly_Cloudy_Day",
+            "Night target 'Partly_Cloudy_Day' of leaf 'Sunny_Day' also has a night target.",
+        ),
+        (
+            "lightning_shower",
+            "Target 'lightning_shower' of leaf 'Sunny_Day' is not a leaf.",
+        ),
+    ),
+)
+def test_check_tree_if_night(modify_tree, expected):
+    """Test that the various possible decision tree problems related to if_night are identified.
+    These are separated out because we need to mark the night leaf as unreachable"""
+    modify_tree["Clear_Night"]["is_unreachable"] = True
+    result = check_tree(modify_tree)
+    assert result == expected
+
+
+@pytest.mark.parametrize(
+    "nodes, expected",
+    (
+        ({"Thunder": 28}, "These leaf categories are used more than once: [28]",),
+        (
+            {"Thunder": 28, "Thunder_Shower_Day": 28},
+            "These leaf categories are used more than once: [28]",
+        ),
+        (
+            {"Thunder": 28, "Heavy_Snow": 26},
+            "These leaf categories are used more than once: [26, 28]",
+        ),
+    ),
+)
+def test_check_tree_duplicate_leaves(nodes, expected):
+    """Test that the various possible leaf duplicates are identified."""
+    tree = wxcode_decision_tree()
+    for node, value in nodes.items():
+        tree[node]["leaf"] = value
+    result = check_tree(tree)
+    assert result == expected
+
+
 def test_check_tree_non_dictionary():
     """Check ValueError is raised if non-dictionary is passed to check_tree."""
     expected = "Decision tree is not a dictionary"