Hello Team,
This is extremely easy to implement, let me guide you.
First see my outputs and final results-
I had an array that had values 1,2,2,3,2 and needed to remove duplicates.
I used the Union() expression and got the final result 1,2,3.

How to do this, first load your array outputs in COMPOSE. My Array outputs are currently residing in array variable called ARRAY

Then Create another COMPOSE and use the following expression -
union(outputs('Compose'),variables('Array'))
Here COMPOSE is the action where the Array was loaded and Array is the name of the Array variable. Please refer the 1st compose action we create you will understand it.
This is all you need to do.
Once this is done, create another COMPOSE and load the outputs of Compose created earlier (the one where we put the expression) to get unique values returned.
If this reply has answered your question or solved your issue, please mark this question as answered. Answered questions helps users in the future who may have the same issue or question quickly find a resolution via search. If you liked my response, please consider giving it a thumbs up. THANKS!