`re.sub` returns duplicated replacement strings in some cases.

# Bug report
```python
re.sub(r'.*', r'x', 'a')   # expect: 'x', actual: 'xx'
re.sub(r'^.*$', r'x', 'a')   # expect: 'x', actual: 'x'
re.sub(r'^.*', r'x', 'a')   # expect: 'x', actual: 'x'
re.sub(r'.*$', r'x', 'a')   # expect: 'x', actual: 'xx'
```

# Environment
- CPython 3.10.6 / Linux
- CPython 3.8.13 / Windows
