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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.

assert ms.rekey_on_member(list_original, key) == expected
    @pytest.mark.parametrize("expected_exception_type, list_original, key, expected", INVALID_ENTRIES)
    def test_fail_rekey_on_member(self, expected_exception_type, list_original, key, expected):
        with pytest.raises(expected_exception_type) as err:
            ms.rekey_on_member(list_original, key)
        assert err.value.message == expected
    def test_duplicate_strategy_overwrite(self):
        list_original = ({'proto': 'eigrp', 'id': 1}, {'proto': 'ospf', 'id': 2}, {'proto': 'eigrp', 'id': 3})
        expected = {'eigrp': {'proto': 'eigrp', 'id': 3}, 'ospf': {'proto': 'ospf', 'id': 2}}
        assert ms.rekey_on_member(list_original, 'proto', duplicates='overwrite') == expected