I've even got a couple self-sown from last year.
Yep, Ruggie, they'll do that. Shed seed (ie those which have been shed naturally from the plant, rather than those kept in a shed), always seem to germinate and grow better than the ones you have deliberately sown. And if you practice crop rotation, as you should, they become annoying weeds.
I always have to remove peas from my tatties or brassicas, because much as I like peas, I never pick them all. And when I remove the remains, some remain to haunt me.
A weed is only a plant growing where you don't want it.
Fitivver, going back to topic, legumes (peas and beans) will keep for several years. Lettuce varieties will usually keep for two. but the germination percentage is reduced, so sow them more thickly. Roots, like parsnips and carrots will not keep. Don't even bother.
The bigger the seed, the better the chances of keeping it for more than a year - that's the rule of thumb.