We are usually able to detect the deletion. Some are too small to see and other technologies can be used, but it is impossible to say at exactly what spot the deletion started and ended. So one individual might have a few more genes deleted than another individual with the 'same' deletion. A duplication is just that, a duplication of a section of a chromosome. A duplication is sometimes referred to as a 'partial trisomy'.
Trisomy refers to three.
Therefore if a duplication exists, that individual has three copies of that area instead of two. In the picture, red arrows point to identical bands on each chromosome. The blue arrow points to a duplication of the band at the red arrow. You can see that the chromosome on the right is longer. A ring chromosome can happen in two ways.