Yes, you can include the same track on multiple single, EP and album releases. For example, you may want to include a previously released single on a new album.
However, if you include an existing track on a new release, you must always upload from the same Ditto account and use the same ISRC code.
You can find the ISRC code of an existing track by scrolling down to ‘My Releases’ on the ‘My Music’ section of your Ditto dashboard. Choose ‘View’ from the dropdown menu next to the track in question and scroll down to Track List to see the associated ISRC codes.