Add filter_non_unique snippet

snippets/filter_non_unique.md

@@ -0,0 +1,17 @@
+---
+title: filter_non_unique
+tags: list,beginner
+---
+
+Filters out the non-unique values in a list.
+
+Use list comprehension and `list.count()` to create a list containing only the unique values.
+
+```py
+def filter_non_unique(lst):
+  return [item for item in lst if lst.count(item) == 1]
+```
+
+```py
+filter_non_unique([1, 2, 2, 3, 4, 4, 5]) # [1, 3, 5]
+```