This was a 'young-adult' SF novel I read in the late 1970's, about the same time I was reading Voorloper by Andre Norton, Madeleine L'engle's Wrinkle In Time books and the Dragonfall 5 young adult novels. so I am not sure if it is actually one of the Dragonfall 5 stories, but I can't confirm it is or isn't by reading available synposes online.
As I recall, the story went something like this...
* SOME SPOILERS BELOW *
A young person and sibling - I think female protag and younger brother, but not sure - is scheduled to visit their aunt and uncle on another planet but to make the voyage in a timely fashion, must utilize this new consciousness transference technology to transmit their consciousness across the cosmos to a host replica body at the destination. After arriving at the destination the protagonist is impressed by their new body and is aware of the only way to tell they are in a replicant body is a small seam, scar or patch on their back, I think near the top of their spine, just below the collar of whatever clothing they wore.
The mystery begins when the protagonist realizes their hosts also sport the same scar when they should legitimately be in their real bodies. The plot thickens when the protag or sibling finds a room of people in stasis when their replicant counterparts are roaming the same planet, and is subsequently caught in the act. Sadly, I do not remember anything else, including the reasoning behind the deception.
I was quite young when I read this novel, so I probably got some of the details mixed up, but would love to know the name and author of the novel if anyone remembers.