According to the section 3.4.6 of the EPUB Publications 3.0 spec, only one <dc:source> element is allowed in the OPF file for an EPUB3 package; it specifies a primary metadata expression. But according to section 3.4.7, it's also possible to use a <meta> element (in the namespace, not the html one...) to specify a primary expression for the same source of the package. So


is not valid per spec but since nothing appears to be said in the spec about uniqueness of definition for primary expressions in the spec

<meta property="dcterms:source">urn:isbn:9780375704025</meta>

is then perfectly valid... Of course, the behaviour of EPUB3 browsing or editing environments in that case is undefined :-(

And guess what? I have in front of me right now a "EPUB3" book having both<dc:source> and <meta property="dcterms:source"> elements. Lovely...