Sorry for the delay on responding, I just got back from a long overdue vacation. The 1919 Mercury Dime shows the doubling in the words "GOD and RUST of TRUST". The last T is TRUST really shows the doubling. Sometimes a doubled die shows up as only thicker letters, but when you have strong separation in the doubling, then you get the heavy interest from collectors. There are several great websites that are mentioned in this thread that will help you understand the difference between a doubled die and machine doubling. Most new collectors struggle at first to differentiate between the two. Once you understand how each is made, it will make a lot more sense.