diff --git a/scripts/gen_rack.py b/scripts/gen_rack.py
index a3e65d5..1b01fc7 100644
--- a/scripts/gen_rack.py
+++ b/scripts/gen_rack.py
@@ -94,3 +94,24 @@ def validate_item(fm: dict) -> None:
         raise SchemaError(
             f"{name}: occupies U{u}..U{u + h - 1}, exceeds {RACK_UNITS}U"
         )
+
+
+def check_overlaps(items: list[dict]) -> None:
+    """Raise if two items share a U on the same face within one rack."""
+    occupied: dict[tuple[str, int], str] = {}
+    for fm in items:
+        face = fm.get("rack_face")
+        if face in ZERO_U_FACES:
+            continue
+        faces = ("front", "rear") if face == "both" else (face,)
+        u = fm["rack_u"]
+        h = fm["u_height"]
+        name = fm.get("hostname", "?")
+        for f in faces:
+            for uu in range(u, u + h):
+                key = (f, uu)
+                if key in occupied:
+                    raise SchemaError(
+                        f"U{uu} {f}: {name} overlaps {occupied[key]}"
+                    )
+                occupied[key] = name
diff --git a/tests/test_gen_rack.py b/tests/test_gen_rack.py
index 724cb59..3829512 100644
--- a/tests/test_gen_rack.py
+++ b/tests/test_gen_rack.py
@@ -40,3 +40,37 @@ def test_validate_accepts_zero_u_rail():
 def test_validate_rejects_missing_units_on_faced_item():
     with pytest.raises(gen_rack.SchemaError):
         gen_rack.validate_item(item(rack_face="front"))
+
+
+def test_overlaps_detects_same_face_overlap():
+    items = [
+        item(hostname="a", rack_u=1, u_height=2, rack_face="front"),
+        item(hostname="b", rack_u=2, u_height=1, rack_face="front"),
+    ]
+    with pytest.raises(gen_rack.SchemaError):
+        gen_rack.check_overlaps(items)
+
+
+def test_overlaps_allows_same_u_different_face():
+    items = [
+        item(hostname="a", rack_u=5, u_height=1, rack_face="front"),
+        item(hostname="b", rack_u=5, u_height=1, rack_face="rear"),
+    ]
+    gen_rack.check_overlaps(items)  # no raise
+
+
+def test_overlaps_both_face_conflicts_with_front():
+    items = [
+        item(hostname="a", rack_u=5, u_height=1, rack_face="both"),
+        item(hostname="b", rack_u=5, u_height=1, rack_face="front"),
+    ]
+    with pytest.raises(gen_rack.SchemaError):
+        gen_rack.check_overlaps(items)
+
+
+def test_overlaps_ignores_zero_u_rails():
+    items = [
+        item(hostname="p1", rack_face="left"),
+        item(hostname="p2", rack_face="left"),
+    ]
+    gen_rack.check_overlaps(items)  # no raise