CCC always examines the files on the destination to determine if they already match those on the source. If you have a volume that is virtually identical to your source, CCC will copy only the items that are different between the two volumes.
Scenario 1: Clone created by another cloning utility
If the software you used previously created a non-proprietary clone of your source to the destination, then CCC will copy only the items that have changed since you created the backup. CCC doesn't care what application you used to copy the files previously, only whether the files match based on name, path, and modification date.
Scenario 2: I replaced my hard drive with an SSD, and now I want to use the HDD as my backup
Whether you cloned your HDD to the SSD or used Migration Assistant to get your data there, the bulk of the data on your HDD and SSD are identical. Once again, CCC doesn't care how the data got there or what application put it there, CCC will copy only the items that are different between the two volumes.
Scenario 3: My backup is in a folder on the destination. Why is CCC recopying everything?
The common use of CCC is to create a bootable clone of your startup disk. To do this, CCC copies all of the stuff from your source volume directly to the destination volume — not into a subfolder, but directly to the destination. At the end of the task, the destination looks exactly like the source. Typically you see "Applications", "Library", "System", and "Users" on the source volume, so that's exactly what you should see on the destination volume.
If your previous backup was placed in a folder, however, then you must instruct CCC to place your backup into that same folder (assuming that's what you want — OS X will not work when placed in a folder on the destination). To do this, choose "Choose a folder" from the Destination menu to select the folder that your backup should be placed into.