Question

what's the cell potential of a copper electrolytic cell if both electrodes are copper? (the e0 value of the copper reaction is 0.34V)

Answer 1

It depends upon the concentration of the electrolyte in each half cell. If each electrode is dipping into a 1 M solution of Cu^2+ then the cell potential will be zero. Look up cell potential in a "concentration cell" on Google or in your text book/notes for a full description.