The first liaison (en un) is optional and rare, the second one (un an) always done, no exception.

Un can be realized as either /ɛ̃/, /œ̃/ or other variants, depending on the region.

The most common pronunciation around me (Paris area) is :

En un an /ɑ̃ ɛ̃n‿ɑ̃/

but the alternative /ɑ̃n‿ɛ̃n‿ɑ̃/ is nevertheless valid.

Note also that the first liaison is expected, and even mandatory in fixed expressions, when the word following un starts with a consonant, so isn’t itself subject to a liaison, e.g.:

En un mot, merci ! /ɑ̃n‿ɛ̃ mo/