Question about checking an array.

[Jiroxys7]

So i'm learning about how all this works. and ive stumbled across a problem.
lets say i'm storing a variable in an array, and i want the script to check to see if they already have that variable in the array before adding it again. (to prevent duplicates)

i've tried a few things such as using if("varthing": in client.varname) or whatever and strcontains wasnt the answer either. what should i be using to check?