Will I receive a stamp from Time Machine or World Explorer that I already have in my collection?

No, when you purchase a random stamp from Time Machine or World Explorer, you will not receive a stamp that you already own in My Stamps. However, it is possible to get a stamp that you have previously collected in your Stamp Collection.

This same rule applies when purchasing stamps from special series, such as the "Castles in Europe". You won't receive duplicate stamps you already own in My Stamps.