1) When you move your mouse cursor over a content block, a pair of buttons will appear near the top of the content box. The "+" will add a block, and the "-" will remove the block the cursor is over.
2) Adding a block will create a copy of the content block underneath the existing one.
NOTE: Deleting the last content block in a section of the email will cause a warning to come up. This is because removing the last content block can create situations where you won't be able to add content. We advise not doing this unless you are certain.