Example: I have two documents from the same collection, both and sharing nearly all characteristics that I want to enter into my Omeka database. How do I duplicate the existing entry so that I don't have to re-enter the shared information and only need to update the new record?
You might want to try the csv import plugin for this purpose. You fill out a spreadsheet, and then upload the metadata all at once to Omeka. It's easy to copy fields that have the same information from one record to another.
Is this still the best answer for the OP's question as of July 2014? Spreadsheets are a good solution if you have 20 records that closely resemble each other, less good if you have two or three. A 'copy record for new' button would be pretty useful. My level of technical understanding prevents my knowing how possible this is to implement, so apologies if it's not do-able using the structures underlying Omeka.
Presently, that is still the best option, if you have the CSV going on. This would be a pretty easy plugin to build, though, if there are any takers out there.
Yes, I support the 'copy record' option. this is especially helpful while creating different collections/iems with same metadata