The Two Graphs

The Two Graphs is a 1950 detective novel by John Rhode, the pen name of the British writer Cecil Street. It is the fiftieth in his long-running series of novels featuring Lancelot Priestley, a Golden Age armchair detective. It was published in America by Dodd Mead under the alternative title Double Identities. Writing in The Observer Maurice Richardson noted a "slight slackening of tension towards the finish but an excellent specimen of Rhode’s later period."

Synopsis
In the Norfolk Broads one of a pair of identical twin brothers drowns, but it is not clear which one. Matters are further complicated when the surviving twin is poisoned.