Can You Burn A CD That Has Already Been Burned?

If the cd is a cdR, no, it can only be burnt once if it's a cdrw, yes, it can be burnt many times.

Yes.
      
If you are using a rewritable compact disk (cd r/w) it is possible to erase the contains of the cd and you can add new data by burning it again. And this can be done again and again with Cd r/w.
       
But If you are using cdr i.e. Compact Disk Retable it can be burnt only once then you cant change the contains of the cd.
      
This had happened with me lots of times that I have burnt a cdr with less than 50mb data and I was able to burn it again but this thing is not promisary.This may happen May not. But you can at
Least takes a chance.

YES....YOU Can burn a CDR that has already been burned......

the simple answer is that u have an option of multisession disk in a burning software like Nero burner.. when a Cdr is burnt with multisession option enabled the same cdr can be written again...
eg; u burnt 50mb data first time with multi session active...it will create a boundry around the session and will leave the remaining portion of that cd as un marked or unused.. for future use or burning data again on to it....

Yes you can but just one thing.... It has to be a cd-rw cd if it  is a cd-R cd then you can only burn it once