Smith adds these functions to the settings 'cog' icon, in the top-right of each matrix block.
To clone a block, select
Clone in the settings menu for a block. This will add a new block at the end of your existing blocks, with the content duplicated. You can only clone a single block at a time.
To copy a block, select
Copy in the settings menu for a block. You can also select multiple blocks to be copied using the checkboxes in the top-left of any block (next to the title). This will save blocks to local storage, for pasting later.
To paste a single block, or multiple blocks, select
Paste in the settings menu for a block. This will paste all copied blocks at the end of your existing blocks.
You can only paste from blocks that were copied from the same Matrix field. However, this will work across entries and sections. You cannot copy blocks from one matrix field to another (the blocks will most likely be different).