If you are loading 9mm 115gr Hornady FMJ RN with say... I dunno... 5.5gr Unique (asking for a friend). You have two pounds of Unique purchased at separate times, possibly a few years, and you are just finishing the first on your progressive... do you....
If you’re really paranoid about it, dump the old powder in with the new, give it a good shake and refill the measure. I’m sure any difference would be negligible, but if you mix a small amount into a whole pound, you’re sure to be safe.