Other forms: dead ringers
A dead ringer is a person who looks just like someone else. Your parents can tell the difference between you and your twin, but to the rest of the world, you're dead ringers for each other.
If your new dog is a dead ringer for one you had years ago, he's nearly identical to good old Spot. This phrase comes from 19th-century horse racing slang. A ringer was a faster horse secretly substituted for a slower one with a similar look in order to cheat people betting on a race. A dead ringer uncannily resembled the original animal, with dead meaning "exact," as in dead center or dead level.