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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.

def _do_fail(e):
        if case_sensitive or attribute:
            raise AnsibleFilterError("Jinja2's unique filter failed and we cannot fall back to Ansible's version "
                                     "as it does not support the parameters supplied", orig_exc=e)
    error = e = None
    try:
        if HAS_UNIQUE:
            c = do_unique(environment, a, case_sensitive=case_sensitive, attribute=attribute)
            if isinstance(a, Hashable):
                c = set(c)
            else:
                c = list(c)
    except TypeError as e:
        error = e
        _do_fail(e)
    except Exception as e:
        error = e
        _do_fail(e)