### 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
for i, line in enumerate(lines):
                    if insertre.search(line):
                        n0 = i
            if n0 is None:
                n0 = len(lines)
            elif insertafter is not None:
                n0 += 1
        elif insertbefore is not None:
            n0 = 0  
        else:
            n0 = len(lines)  
    elif n0 < n1:
        lines[n0:n1 + 1] = []
    else:
        lines[n1:n0 + 1] = []
        n0 = n1
    
    if n0 > 0:
        if not lines[n0 - 1].endswith(b(os.linesep)):
            lines[n0 - 1] += b(os.linesep)
    lines[n0:n0] = blocklines
    if lines:
        result = b''.join(lines)
    else:
        result = b''
    if module._diff:
        diff['after'] = result
    if original == result:
        msg = ''
        changed = False