JabRef 4.0.0-dev--snapshot--2017-04-03--master--b45575649
Windows 7 6.1 amd64
Java 1.8.0_121

Steps to reproduce:
0. open PMID 21497511 on Pubmed

1. use Jabfox/Zotero for importing
2. While in Jabref, click on the DOI symbol to open the paper on the journals home page.
3. Import the article via Jabfox/Zotero
4. While Authors, Journal, Volume, pages, DOI are all equal between the two entrys, the article, however, is not labelled as duplicate in Jabrefs import dialogue and furthermore not found with Quality->Find Duplicates.

This is indeed very strange. I wonder what Jabref now considers as duplicate of an entry.

Put the excerpt of the log file here