Will doubling the yeast cause any real problems? I have an extra Safeale US05, and was wondering about throwing it in my next brew (also uses us05). Any harm? not worth it?
As rmyurick has said, it might be a good idea, but it really depends on the OG of your recipe. Generally 1 pack of dry yeast is fine unless your getting into big beers. If you don't need it, why bother putting it in there, just save it for your next batch.
There is about 20 billion yeast cells per gram of dry yeast (US-05) properly rehydrated yeast loses about 25% viability so you are left with approximately 150 billion cells, which is fine for a standard gravity beer. However a 1.069 beer will require around 300 billion cells so pitching two packs of properly rehydrated yeast is a good idea.