Use it! I'd snip off a piece of the taped area after thoroughly sanitizing the outside of the yeast envelope. Then hydrate the yeast.
You could shake out any amount, eyeballing the volume, or into a sanitized measuring spoon, and reseal for the next time. Fold over the snipped corner and tape it off. I would store it in the freezer though, inside a ziplock/freezer bag.
There's a thread here on sealing hops where a member also repackages 1lb bricks of Safale S-05 into pre-measured amounts and stores them heat-sealed plastic pouches. He acknowledges the small potential risk of infection but claims to have had no problems so far.
Grossly overpitching by using a whole packet (11 grams) in a gallon of wort will likely give you off-flavors.