Riders from Nowhere. Trilogy

In 1967, a science fiction novel by Alexander and Sergey Abramov, 'Riders from Nowhere', father and son, was published in the Moscow magazine 'Smena'. The novel was an explosion in the perception of readers at the time. A Soviet polar expedition. Blocks of ice, cut from the Arctic ice cap by unknown hands and sent who knows where. Mysterious pink clouds over the planet that do not obey the laws of physics. Doubles of people appearing and disappearing without a trace. No, alas, not without a trace: doubles start appearing all over the earth, moreover, phantom cities emerge where these human duplicates live and create their own history. An alien intelligence is researching humanity. What its purpose is — is unknown…
A year later, a continuation of 'Riders' appeared — the novel 'Paradise without Memory', and ten years later the conclusion of the trilogy — the novel 'Silver Variant'.
In the first book, the heroes only begin to approach contact with the incomprehensible extraterrestrial force that visited our planet, while in the following novels of the trilogy they enter into a decisive struggle with the consequences of a blind experiment imposed on us by outsiders from beyond.
According to some readers, 'The Doomed City' (1975) by the Strugatsky brothers is a replica of the novel by A. and S. Abramov 'Paradise without Memory'. Perhaps, we will not argue.