First of all, welcome to the community.
I moved your thread to Support > Basics & Interface, as it’s not really a tutorial or a tip.
As you’re new, I’d advise you to read the Forums’ FAQ.
Since you probably can’t post any pictures yet, it’s difficult to understand what you’re refering to with “these”.
So I’d guess that you were in Edit Mode when you duplicated a mesh. If so, you can separate mesh parts into new objects, by selecting the parts you want to separate in Edit Mode, and then press P (by selection).