Is a preprint considered as dual publication?
Nature encourages the use of preprints, as noted on their journal's policy page: 


  Nature Research journals encourage posting of preprints of primary research manuscripts on preprint servers, authors’ or institutional websites, and open communications between researchers whether on community preprint servers or preprint commenting platforms. Preprints are defined as an author’s version of a research manuscript prior to formal peer review at a journal, which is deposited on a public server (as described in Preprints for the life sciences. Science 352, 899–901; 2016); preprints may be posted at any time during the peer review process. Posting of preprints is not considered prior publication and will not jeopardize consideration at Nature Research journals. Manuscripts posted on preprint servers will not be taken into account when determining the advance provided by a study under consideration at a Nature Research journal.


The webpage then proceeds  to provide the details for their policy.