On any profile, if you click the little magnifying glass symbol beside the name, it does a search for duplicates. However, it doesn't necessarily catch everything (people spell names wrong, etc.). If I'm doing a thorough search I usually go to the surname page (eg. http://www.wikitree.com/genealogy/Smith ), and sort alphabetically.
I think I've answered your question, but as a side note, there is a Compare tool, but that's something different. :) On the merge screen, before merging, matching, or rejecting, you can compare, which shows you two profiles side by side.