### 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 = AnsibleModule(
        argument_spec=dict(
            path=dict(type='path', required=True, aliases=['dest', 'destfile', 'name']),
            state=dict(type='str', default='present', choices=['absent', 'present']),
            marker=dict(type='str', default='# {mark} ANSIBLE MANAGED BLOCK'),
            block=dict(type='str', default='', aliases=['content']),
            insertafter=dict(type='str'),
            insertbefore=dict(type='str'),
            create=dict(type='bool', default=False),
            backup=dict(type='bool', default=False),
            validate=dict(type='str'),
            marker_begin=dict(type='str', default='BEGIN'),
            marker_end=dict(type='str', default='END'),
        ),
        mutually_exclusive=[['insertbefore', 'insertafter']],
        add_file_common_args=True,
        supports_check_mode=True
    )
    params = module.params
    path = params['path']
    if os.path.isdir(path):
        module.fail_json(rc=256,
                         msg='Path %s is a directory !' % path)
    path_exists = os.path.exists(path)
    if not path_exists:
        if not module.boolean(params['create']):