1 files changed, 23 insertions, 0 deletions
diff --git a/setuptools/_itertools.py b/setuptools/_itertools.py
new file mode 100644
index 00000000..b8bf6d21
--- /dev/null
+++ b/setuptools/_itertools.py
@@ -0,0 +1,23 @@
+from setuptools.extern.more_itertools import consume  # noqa: F401
+
+
+# copied from jaraco.itertools 6.1
+def ensure_unique(iterable, key=lambda x: x):
+    """
+    Wrap an iterable to raise a ValueError if non-unique values are encountered.
+
+    >>> list(ensure_unique('abc'))
+    ['a', 'b', 'c']
+    >>> consume(ensure_unique('abca'))
+    Traceback (most recent call last):
+    ...
+    ValueError: Duplicate element 'a' encountered.
+    """
+    seen = set()
+    seen_add = seen.add
+    for element in iterable:
+        k = key(element)
+        if k in seen:
+            raise ValueError(f"Duplicate element {element!r} encountered.")
+        seen_add(k)
+        yield element