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##### SUMMARY
<!--- Explain the problem briefly below -->
`unique` filter does not respect the order and gives non-idempotent results

##### ISSUE TYPE
- Bug Report


##### COMPONENT NAME
lib/ansible/plugins/filter/mathstuff.py


##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible 2.8.5
```

##### STEPS TO REPRODUCE
<!--- Describe exactly how to reproduce the problem, using a minimal test-case -->
I have the following statement in my template 

```jinja2
{% for host in url_list | map('urlsplit', 'hostname') | unique %}
    upstream {{ host }} {
        server {{ host }}:443;
        keepalive 20;
        keepalive_requests 500;
        keepalive_timeout 30s;
    }
{% endfor %}
```

which basically extracts unique host names from some url list and declares upstream server for each. 

##### EXPECTED RESULTS
The order of hosts is at least consistent between runs. 


##### ACTUAL RESULTS
The order of hosts is different for each run. Unexpected handlers are triggered.

''' Ansible math jinja2 filters '''
    def filters(self):
        filters = {
            
            'min': min,
            'max': max,
            
            'log': logarithm,
            'pow': power,
            'root': inversepower,
            
            'unique': unique,
            'intersect': intersect,
            'difference': difference,
            'symmetric_difference': symmetric_difference,
            'union': union,
            
            'product': itertools.product,
            'permutations': itertools.permutations,
            'combinations': itertools.combinations,
            
            'human_readable': human_readable,
            'human_to_bytes': human_to_bytes,
            'rekey_on_member': rekey_on_member,
            
            'zip': zip,
            'zip_longest': zip_longest,
        }
        return filters