feat(api): raise better error on column name collision in joins

ibis/expr/operations/relations.py

@@ -648,7 +648,25 @@ def _dedup_join_columns(expr, lname: str, rname: str):
         for column in right.columns
         if column not in equal
     ]
-    return expr.select(left_projections + right_projections)
+    projections = left_projections + right_projections
+
+    # Certain configurations can result in the renamed columns still colliding,
+    # here we check for duplicates again, and raise a nicer error message if
+    # any exist.
+    seen = set()
+    collisions = set()
+    for column in projections:
+        name = column.get_name()
+        if name in seen:
+            collisions.add(name)
+        seen.add(name)
+    if collisions:
+        raise com.IntegrityError(
+            f"Joining with `lname={lname!r}, rname={rname!r}` resulted in multiple "
+            f"columns mapping to the following names `{sorted(collisions)}`. Please "
+            f"adjust `lname` and/or `rname` accordingly"
+        )
+    return expr.select(projections)
 
 
 public(ExistsSubquery=ExistsSubquery, NotExistsSubquery=NotExistsSubquery)

ibis/tests/expr/test_table.py

@@ -1592,6 +1592,18 @@ def test_join_lname_rname(how):
     assert expr.columns == ["left_id", "first_name", "right_id", "last_name"]
 
 
+def test_join_lname_rname_still_collide():
+    t1 = ibis.table({"id": "int64", "col1": "int64", "col2": "int64"})
+    t2 = ibis.table({"id": "int64", "col1": "int64", "col2": "int64"})
+    t3 = ibis.table({"id": "int64", "col1": "int64", "col2": "int64"})
+
+    with pytest.raises(com.IntegrityError) as rec:
+        t1.left_join(t2, "id").left_join(t3, "id")
+
+    assert "`['col1_right', 'col2_right', 'id_right']`" in str(rec.value)
+    assert "`lname='', rname='{name}_right'`" in str(rec.value)
+
+
 def test_drop():
     t = ibis.table(dict.fromkeys("abcd", "int"))