Pre WW1 England assuming another identity

A Lord returns home to his wife and estate after an extended stay in Africa. It is eventually revealed that he met a man while on safari that looked very much like him. I can't remember if he was actually the Lord or the imposter, but he was helping the government to reveal German spies in England. It was a Readers Digest condensed book. I'd like to find the full version.

Have you tried looking through a list of Reader's Digest condensed books to see if any titles ring a bell?

The Scapegoat by Daphne du Maurier (an RD condensed book) involves doppelgangers who swap identities, but it's set after WW2.