Merge two arrays of ElementModels?
Yep, there's a great mystery about how the in containment operator actually works with objects. Twig Docs say it's possible:


  You can use this filter to perform a containment test on strings, arrays, or objects implementing the Traversable interface.


but I also tried this (multiple times) and did't get it working. I eventually always ended up using a workaround. In your case I'd do the following and use the entry's ID in the conditional:

{# Get arrays of entries #}
{% set entriesA = craft.entries(paramsA).find() %}
{% set entriesB = craft.entries(paramsB).find() %}

{# Prepare merge #}
{% set entriesBIds = craft.entries(paramsB).ids() %}
{% set allEntries = entriesB %}

{# Merge arrays #}
{% for entry in entriesA %}

    {# Don't merge duplicates #}
    {% if entry.id not in entriesBIds %}
        {% set allEntries = allEntries|merge([entry]) %}
    {% endif %}

{% endfor %}