This New Crib Ad from IKEA Actually Doubles as a Pregnancy Test — Like, for Real
The sentence is came from an article, I have no idea what does the word "doubles" mean there.
Doubles in this context basically just means that something serves two purposes/functions at the same time.
For example if you have a couch that could be converted into a bed, that couch DOUBLES as a bed. If you have a pen, that turns into a knife, the pen DOUBLES as a knife.
The ad you are talking about "doubles" because it has dual function as an ad and also a pregnancy test at the same time.
This meaning comes from the meaning of double, meaning a pair of things.