### Summary


we config the domain info with the `/etc/hosts` file, we monitor our service and change the domain with bash script like this:
```
ansible 127.0.0.1 -m blockinfile -a "block='192.168.1.1 test.company.com' marker='# {mark} : test block' path=/etc/hosts"
```
BUT , occasionally the file `/etc/hosts` contains multiple block. I expected that there should be only one block.

I have read the source code `lib/ansible/modules/files/blockinfile.py` and find nothing suspicious.


### Issue Type
~Bug  Report~
- Docs Report

### Component Name

blockinfile

### Ansible Version

```console
$ ansible --version
ansible 2.7.5.post0
  config file = /etc/ansible/ansible.cfg
  configured module search path = [u'/root/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/lib/python2.7/site-packages/ansible
  executable location = /usr/bin/ansible
  python version = 2.7.5 (default, Aug  7 2019, 00:51:29) [GCC 4.8.5 20150623 (Red Hat 4.8.5-39)]
```


### Configuration

```console
$ ansible-config dump --only-changed
COMMAND_WARNINGS(/etc/ansible/ansible.cfg) = False
HOST_KEY_CHECKING(/etc/ansible/ansible.cfg) = False
```


### OS / Environment

CentOS-7.6

### Steps to Reproduce

<!--- Paste example playbooks or commands between quotes below -->
```yaml (paste below)
ansible 127.0.0.1 -m blockinfile -a "block='192.168.1.1 test.company.com' marker='# {mark} : test block' path=/etc/hosts"
```


### Expected Results

```console
# BEGIN : test block
192.168.1.1 test.company.com
# END : test block
```

### Actual Results

```console
# BEGIN : test block
192.168.1.1 test.company.com
# END : test block
# BEGIN : test block
192.168.1.2 test.company.com
# END : test block
```


### Code of Conduct

- [X] I agree to follow the Ansible Code of Conduct
module.fail_json(rc=257,
                             msg='Path %s does not exist !' % path)
        destpath = os.path.dirname(path)
        if not os.path.exists(destpath) and not module.check_mode:
            try:
                os.makedirs(destpath)
            except Exception as e:
                module.fail_json(msg='Error creating %s Error code: %s Error description: %s' % (destpath, e[0], e[1]))
        original = None
        lines = []
    else:
        with open(path, 'rb') as f:
            original = f.read()
        lines = original.splitlines(True)
    diff = {'before': '',
            'after': '',
            'before_header': '%s (content)' % path,
            'after_header': '%s (content)' % path}
    if module._diff and original:
        diff['before'] = original
    insertbefore = params['insertbefore']
    insertafter = params['insertafter']
    block = to_bytes(params['block'])
    marker = to_bytes(params['marker'])
    present = params['state'] == 'present'
    if not present and not path_exists:
        module.exit_json(changed=False, msg="File %s not present" % path)
    if insertbefore is None and insertafter is None:
        insertafter = 'EOF'
    if insertafter not in (None, 'EOF'):