Yes it can produce some undesirable flavors to overpitch.
Are you planning to do different worts with the same yeast? Or the same wort with different yeast?
If using dry yeast I would recommend rehydrating in a given volume (100ml) and then use 20ml of that for each 1gal batch. This way 1 sachet of dry yeast provides 5 individual 1gal batches.
If you're looking to use different yeasts then unfortunately this is still the best way, but you'll be pouring 80% of each yeast down the drain. OR preferably, after rehydrating make a starter from them and then package and refrigerate them for another brew.