I've stumbled on a case where two different publishers have issued different DOIs for the same articles. In this case, Springer and J-State both publish the Japanese Journal of Ichthyology (ISSN 0021-5090). The following article: is published by Springer with the DOI http://dx.doi.org/10.1007/BF02914322, and this DOI is registered with CrossRef. J-Stage publish the same article, with the DOI (http://dx.doi.org/10.11369/jji1950.36.196).