This is just D doublesharp, which is enharmonic to E.
The trick is that key signatures are not additive. In other words, any accidental added to a pitch is considered to be its own construct, not something in addition to what's already given in the key signature.
As such, this is not D♯ that is then sharped twice again, but rather just D doublesharp.
I would like to add to Richard's answer that if you wanted to "undo" the double-sharp (maybe you need a D sharp later within the same bar) you would have to put both a natural sign and a sharp sign in sequence before the note head. For example, take a look at bar 46 from the Prelude in C-sharp Minor, BWV 873 from the Well-Tempered Clavier, Vol. II, by Johann Sebastian Bach (BGA XIV, ed. 1866): The key signature requires an F sharp (here it's marked on the lower F in the treble, this being an old-fashioned edition), an F double-sharp is needed in the top voice, and then the alteration is canceled on the next F by means of a natural-sharp combination. You can think of the "X" symbol on the first F standing for two sharps, and the natural sign on the second F canceling only the first sharp, while the second sharp is reiterated for good measure.
The same should be done for double-flats.