Fixed #16609 -- Fixed duplicate admin results when searching nested M2M relations.

This was fixed earlier but only when the M2M relation was at the first
level on the object. This commit fixes the issue even when the M2M is
at deeper levels, such as behind a foreign key.

django/contrib/admin/utils.py

@@ -10,6 +10,7 @@ from django.core.urlresolvers import NoReverseMatch, reverse
 from django.db import models
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.deletion import Collector
+from django.db.models.sql.constants import QUERY_TERMS
 from django.forms.forms import pretty_name
 from django.utils import formats, six, timezone
 from django.utils.encoding import force_str, force_text, smart_text
@@ -22,10 +23,20 @@ def lookup_needs_distinct(opts, lookup_path):
     """
     Returns True if 'distinct()' should be used to query the given lookup path.
     """
-    field_name = lookup_path.split('__', 1)[0]
-    field = opts.get_field(field_name)
-    if hasattr(field, 'get_path_info') and any(path.m2m for path in field.get_path_info()):
-        return True
+    lookup_fields = lookup_path.split('__')
+    # Remove the last item of the lookup path if it is a query term
+    if lookup_fields[-1] in QUERY_TERMS:
+        lookup_fields = lookup_fields[:-1]
+    # Now go through the fields (following all relations) and look for an m2m
+    for field_name in lookup_fields:
+        field = opts.get_field(field_name)
+        if hasattr(field, 'get_path_info'):
+            # This field is a relation, update opts to follow the relation
+            path_info = field.get_path_info()
+            opts = path_info[-1].to_opts
+            if any(path.m2m for path in path_info):
+                # This field is a m2m relation so we know we need to call distinct
+                return True
     return False