Since we have not yet built a “copy” button for manually adding transactions, you can easily use the existing “Split” feature as a workaround. Just use the checkbox at far left to select the transaction you’d like to copy. Then click the “Split” button at upper right.
Use the pop-up window to make as many copies of the original transaction as you’d like, then adjust all the amounts to the original full amount of the income or expense. The pop-up will allow you to save your edits even if the amounts sum up to more (or less) that the original transaction.