It’s “an unique thing” ok? The indefinite article before a vowel sound or a soft aspirate becomes “an“. With an n. An hypothesis. An honourable mention. An indefinite article in fact. An apple and an apron. An historical and linguistic anomaly.
I don’t care if you say something is “almost unique”; that’s a valid use of a qualifier. I don’t care if you say it is “nearly unique” though I’d wince with other people’s referred pain at “very nearly unique”. But if you say something is “a unique thing” I will, without a doubt, loose it. Entirely.
It won’t be pleasant.