Yes, you can reuse fake eyelashes. They should be in a good condition for using up to 4 or 5 times for as long as the glue is properly maintained. Just make sure you are extra cautious when removing them so as not to put pressure on them. Always replace them in their packet while not in use.
I seriously don't think reusing fake eyelashes is the best idea. Sure, it's possible, but say if you're being judged by a cheer or dance judge, you don't want them to fall off. That would be deducted points. I am in Platinum Dance Compitions, and I have a fresh pair of fake eye lashes for each compitions. However, have you ever been to the make up department Sephora? Their brand of mascara works great and half the time I don't have to use fake eyelashes. Just a sugestion.
You can reuse the false lashes as long as they are clean. You don't want to risk an eye infection. They are super cheap and you might just want to stock up on them so you don't have to keep re-using them. (Sometimes they start to look tired after 1 or 2 uses.) I do Lash Extensions at my salon and must say that these are def. something you should look into. These type of lashes you cannot reuse. They bond to your lash one by one using a medical grade glue and are semi permanent. They last between 4-6 weeks. And need to be filled every 3-5 weeks. Check up on Novalash Eyelash Extensions! They are gorgeous, natural looking and you don't have to worry about all that glue and mess at home!
I've read up on fake eyelashes and from what I learned, they are a one-time use item. The glue can collect bacteria in just a few hours, which may lead to eye infections. Reusing them more than once isn't a good idea. Here is a website with the pros and cons of false eyelashes.