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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.

if isinstance(a, Hashable) and isinstance(b, Hashable):
        c = set(a) ^ set(b)
    else:
        isect = intersect(environment, a, b)
        c = [x for x in union(environment, a, b) if x not in isect]
    return c
@environmentfilter
def union(environment, a, b):
    if isinstance(a, Hashable) and isinstance(b, Hashable):
        c = set(a) | set(b)
    else:
        c = unique(environment, a + b)
    return c
@environmentfilter
def min(environment, a, **kwargs):
    if HAS_MIN_MAX:
        return do_min(environment, a, **kwargs)
    else:
        if kwargs:
            raise AnsibleFilterError("Ansible's min filter does not support any keyword arguments. "
                                     "You need Jinja2 2.10 or later that provides their version of the filter.")
        _min = __builtins__.get('min')
        return _min(a)
@environmentfilter
def max(environment, a, **kwargs):