For some people applying fake eyelashes is mission impossible. Others tend to go a bit overboard and wear them everyday.
 Like always in life, moderation is the key and that goes for wearing fake eyelashes as well. From time to time it's nice to pun them on and add a bit more to your look. 
How to do it without looking like you have double lids and lashes? There are two important things to know when it comes to putting on false lashes. 
First is pretty self explanatory: it takes practice. In any activity invented by humans practice makes perfection and this is no exception. So, don't give up first couple times when you make a mess and end up gluing your top and bottom lashes together (this is pretty drastic example, it almost never happens). 
Other thing is the glue. You have to use good quality glue if you want your false lashes to look nice and stay on. I think that only eyelash glue worth buying is DUO Eyelash Adhesive
There are four kinds: Individual lash adhesive, Brush on adhesive and two Striplash adhesives (one dries clear and other one black). I like Striplash adhesive that dries clear the best. Adhesive that dries black can look odd if you have neutral colour shadow and no eyeliner on your lids. Also, if you accidentally get it on some other face area you'll have to remove it and risk removing the makeup. Clear one will just stay on there and won't be visible.
Amazing thing about this glue is that it's equally easy to apply and later remove the lashes. It wont rip your natural lashes out (I know lot of people are concerned about that) and is very gentle to the lids. This adhesive never irritated my eyes (I have pretty sensitive eyes and I wear contacts).