Problem) how to copy a site in SharePoint 2013 ?
 or
How to clone a site in sharepoint 2013 ?
or

Clone a SharePoint 2010 Site using PowerShell ?

or

 copy a sitecollection in SharePoint 2013 ?

or

How to clone a sitecollection in sharepoint 2013 ?

Solution)   To clone a existing sahrepoint site we use power shell command to workaround this .
step1:select the site you want to clone. example:http://site1/
step 2: make a backup of the current site using power shell 
command: 
Backup-SPSite -Identity "http://Site1" -Path "C:\backups\site1.bak" -Force -Confirm:$False

after creating site collection backup now create a new site collection using sharepoint central admin:
steps:
1) goto central admin
2) select manage web application .
3)create new site collection .
4)give site collection a name  ex: http://site2

Now run second POWER SHELL COMMAND : 

Restore-SPSite "http://site2" -Path C:\backups\site1.bak -Force -Confirm:$False                

replace site1, site2 with your  old site and new site urls respectively .

your site might take some time to load when you open you site2 for the first time.