When a Book Has Multiple Titles

As a selector and reader, I often feel flustered at the fact that a so-called new book turns out to be something so familiar that I seriously suspect either I have had a relapse of memory or writers are guilty of plagiarism. They are two recent instances, namely,

Karin Fossum's Eva's eye (2013 US edition) is the same as In the darkness (2012 UK edition)
Jussi Adler-Olsen's The keeper of lost causes (2011 US edition) is the same as Mercy (2011 UK edition).

The latter especially threw me into a mental torture like Merete Lynggaard, the kidnapped victim, for the last few weeks. Last night before going to bed I decided to end my misery once and for all: I had to confirm that The keeper of lost causes was Mercy. Thanks to Amazon, my confirmation was finally proved positive, and I rewarded myself with M.C. Beaton's Something borrowed, someone dead for a bed reading.