I have purchased the Lumia 640 xl online and paid but upon delivery it was a single sim with a second space that looks like a sim space but it seems that it is not. But when i read the details online Lumia 640 xl is a dual sim. So my question is : Is the lumia 640 xl single sim fake ?
The Lumia 640 and 640 XL are available in both single- and dual-SIM variants and the empty space is probably because they use the same chassis for both variants. Also, I see little reason in trying to "fake" this fairly low-end and not particularly popular phone (unlike, say, iPhones or Android flagships).
When researching the specifications of a phone, check multiple sites, including the manufacturer's website. Reviewers often just publish the specifications of the particular device they were reviewing, which could happen to be any available variant.