Fix doubled-word typos in docs and comments

[zaveshaa]

Found four doubled-word typos with a simple regex scan:

1. Doc/library/difflib.rst: "Diffs can be produced in in various formats"
2. Lib/multiprocessing/resource_tracker.py (comment above the Base64 encoding): "we encode encode the raw name bytes"
3. Lib/test/test_exception_group.py (LeafGeneratorTest comment): "Is is also used below as a test utility" → "It is also used"
4. Lib/test/test_unittest/testmock/testhelpers.py: "The return value isn't isn't callable"

PR to follow.

Linked PRs

• gh-156226: Fix doubled-word typos in the difflib documetation #156227