First of all, I shouldn't really have mentioned the classes in the same breath as dual fulfillments, as they're somewhat different subjects, but just to explain the classes a bit more, take a look at this thread: http://www.jehovahs-witness.net/jw/friends/172094/1/WHATEVER-HAPPEND-TO-THE-JONADAB-CLASS
The most prominent prophecy that the JWs give a dual fulfillment to must be the warning by Jesus about the digusting thing in the holy place. This was initially fulfilled in 66-70CE, and is to be fulfilled again in the future when the UN moves against false religion. Here's another one:
http://www.jehovahs-witness.net/watchtower/beliefs/89109/1/Nebochad-and-the-Gentle-times
Here's what the Society wrote on the general subject, with some examples:
*** it-2 p. 693 Prophecy ***
Multiple fulfillment. The use made of prophecy by Jesus and his disciples shows that a predictive prophecy may have more than one fulfillment, as when Paul referred to Habakkuk’s prophecy, originally fulfilled in Babylon’s desolation of Judah, and applied it in his day. (Hab 1:5, 6; Ac 13:40, 41) Jesus showed that Daniel’s prophecy concerning “the disgusting thing that is causing desolation” was due for fulfillment in the generation then living; yet Daniel’s prophecy also connects “the disgusting thing” causing desolation with the “time of the end.” (Da 9:27; 11:31-35; Mt 24:15, 16) Biblical evidence shows that when Michael ‘stands up,’ this signifies that Jesus Christ takes action as king on behalf of Jehovah’s servants. (Da 12:1; see MICHAEL No. 1.) Jesus’ own prophecy regarding the conclusion of the system of things likewise includes mention of his coming in Kingdom power, which did not take place in the first century C.E. (Mt 24:29, 30; Lu 21:25-32) This indicates a dual fulfillment. Hence, in discussing the matter of double fulfillment of prophecy, M’Clintock and Strong’s Cyclopaedia (1894, Vol. VIII, p. 635) comments: “This view of the fulfilment of prophecy seems necessary for the explanation of our Lord’s prediction on the Mount, relating at once to the fall of Jerusalem and to the end of the Christian dispensation.”
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These prophecies are sometimes said by the Society to have a "minor" and "major" fulfillment to indicate a difference in scale, e.g. the destruction of Jerusalem in 70CE vs. the destruction of all false religion in the Great Tribulation. There are also prophetic characters and prefigurements, like Jonah spending three days in the whale prefiguring Jesus' three days in the grave. And so on and so forth. We're basically looking at a situation where people with too much time on their hands are trying to wring as much meaning out of ancient writings as possible. This isn't unique to the Witnesses; they inherited the tendency from earlier spiritual writers in the 1800s.
