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 http://www.idpf.org/2007/opf namespace, not the html one...) to specify a primary expression for the same source of the package. So

<dc:source>urn:isbn:9780375704024</dc:source>
<dc:source>urn:isbn:9780375704025</dc:source>

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

<dc:source>urn:isbn:9780375704024</dc:source>
<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...