Quick and dirty way : 

    =UNIQUE(ARRAYFORMULA(TRANSPOSE(SPLIT(CONCATENATE(A1:F12&"🐥"),"🐥"))))

[(Source : organizing google-sheets data range into single column)][1]  
However, care must be taken not to exceed 50k characters when using `concatenate`. If the case arises, simply break the formula into several pieces or use arrays. 

    =unique({A1:A10;B1:B10;C1:C10})

  [1]: https://webapps.stackexchange.com/questions/115358/organizing-google-sheets-data-range-into-single-column


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Edit : I have just learned that this problem can now be solved in an even simpler way. `=unique(flatten(A1:F12))`