Initial ownership of Copyright
Copyright Act §13 (1):
...the author of a work shall be the first owner of the copyright...
Where the author of a work was in the employment [...] and the work was made in the course of his employment by that person, the person by whom the author was employed shall, in the absence of any agreement to the contrary, be the first owner of the copyright
The "author" may be a joint author. Joint authorship means (§2):
a work produced by the collaboration of two or more authors in which the contribution of one author is not distinct from the contribution of the other author or authors
If the authors did not intend to create a joint work, but rather just contributed independent parts to a work, it is considered a "collective work", with each author maintaining copyright only in those parts that are originally theirs. The Copyright Act defines a collective work to be (among a few other things):
any work written in distinct parts by different authors, or in which works or parts of works of different authors are incorporated
Assignment and licencing
§13(4)-(7) The owner can transfer in whole or in part the copyright to another party. The owner can also give a licence to interest in any of the rights protected by copyright rather than an assignment. Licencing is different than assignment (see Euro-Excellence v. Kraft). In the case of partial transfer, for the purposes of the Copyright Act, the assignee is treated as the owner of the copyright with respect to the assigned rights.
Is there a way to share the copyright, e.g., for a natural person to assign the copyright to some other party on a shared basis, such that both parties could claim the copyright at the same time on the same piece of work?
This was partially tested in Euro-Excellence v. Kraft. The issue in that case was that Kraft Canada was sold an exclusive licence to use some work related to the Toblerone brand in Canada. Euro-Excellence imported into Canada and sold products that used the copyrighted work (produced with authorization in Europe). One question was whether Kraft, an exclusive licensee, could bring a copyright infringement suit. The court recognized that "Where the owner of a copyright has granted an exclusive licence, it has temporarily granted a proprietary interest in the copyright itself to the exclusive licensee", citing Copyright Act §13(7). The act gives "the exclusive licensee the right to invoke the Act for copyright infringement not only against third parties, but also against the owner-licensor".
In Canada, splitting of Copyright can happen in a few situations:
- joint authorship, which is more a sharing rather than a splitting of the right,
- collective works, which gives each contributor copyright only to their contributions,
- partial transfer (for which the statute deems each party to be "owner" only with respect to the rights they retain), and
- licencing (which isn't really ownership, but does give some rights to a party who would otherwise not have them, including, in the case of exclusive licencing, the right to invoke the act for copyright infringement).
I wouldn't call any of these situations "duplication". But, that's just a word choice.
Your linked example appears to be either a work of joint authorship or a collective work, but we lack the information needed to make that determination. Also, the declaration of copyright ownership in your linked example shouldn't be taken as fact. Who actually owns the copyright is not dependent on what is claimed in the file.
In the US
Initial ownership, joint authorship, collective works, and partial transfer are largely the same in the US as in Canada. See 17 U.S.C. 201.