Why is this twig statement behaving weird?

Question

I just run in a weird behaviour in twig.

I've just used a for() over an array that hasn't any keys set but one, which was added into the array before. For test cases, just assume this php array:

$array = array (
    0 => 'test1',
    1 => 'test2',
    'someKey' => 'test3'
)

So, I did this in twig:

{% for key, value in array %}
    {% if key == 'someKey' %}
        {{ 'something special happend' }}
    {% else %}
        {{ 'how boring' }}
    {% endif %}
{% endfor %}

What totally caught me off-guard was the fact, that something special happend has been called twice.

After some searching I've found that this case was also true for the key 0.

So I tried following:

{{ dump (key == 'someKey') }}

The result was

1. true
2. false
3. true

So I did another test:

{{ dump(key|lower == 'somekey') }}

The result was

1. false
2. false
3. true

Since I wanted to be sure, this was my last test

{{ dump(0 == 'somekey') }}
{{ dump(0|lower == 'somekey') }}

The result was

1. true
2. false

Why is this behaviour like it is? I can't explain it.

PS: I know by know that I can use the same as operator, but I'm just curious how this works.