If you want to copy a line and paste in the same file you can simply run:
Which means copy line number 10 and put it here (. means current line). You can pass any number instead dot of course.
Vim also has a shorter synonym for a copy command which is just t, so you can run:
If you're using relativenumber you can use them too:
At last, but not least - if you don't like numbers, you can just search!