[1.5.x] Fixed #19423 -- Prevented ModelAdmin sharing widgets due to formfield_overrides

Thanks joebuyer at manycycles.com for the report and Simon Charette
for the review.
Backport of 04e6542b5 from master.

django/contrib/admin/options.py

@@ -1,3 +1,4 @@
+import copy
 from functools import update_wrapper, partial
 import warnings
 
@@ -130,7 +131,7 @@ class BaseModelAdmin(six.with_metaclass(forms.MediaDefiningClass)):
         # passed to formfield_for_dbfield override the defaults.
         for klass in db_field.__class__.mro():
             if klass in self.formfield_overrides:
-                kwargs = dict(self.formfield_overrides[klass], **kwargs)
+                kwargs = dict(copy.deepcopy(self.formfield_overrides[klass]), **kwargs)
                 return db_field.formfield(**kwargs)
 
         # For any other type of field, just call its formfield() method.